I'm running Firefox 82.0.3 (64-bit) in Linux(The OS shouldn't matter, only the browser, but just in case).

I rarely make use of "Watch Later" but I just tried it and it seemed to work okay here.

Maybe clearing cache would fix it?

Not what you asked for but I use a site specific cookie clearer, mostly to clear cookies set by a couple newspaper sites. (They limit the number of free articles but use cookies to keep count).

It can also be used when leaving Facebook.

https://addons.mozilla.org/en-US/firefox/addon/remove-cookies-button-ver1x/

There are also some adblock filters that I picked up a long time ago. Not sure how useful they are but they haven't broken anything.

||facebook.com$domain=~www.facebook.com
||facebook.net$domain=~www.facebook.com
||fbcdn.net$domain=~facebook.com

The idea being to allow content from Facebook to load only if you're actually on a Facebook domain.

ETA: Just for the heck of it...

Testing whether the above filters block Facebook content from downloading or just hide it...

Newspaper site that uses the Facebook Comments Plugin:

With my adblocker disabled: (not using the above filters)

With my adblocker enabled:

So, even for folks that don't care whether Facebook knows what other pages you frequent, the filters still save bandwidth and decrease page load time.

Of course if you wanted to read the comments, you could disable your adblocker and do a refresh.

I've had no trouble with YouTube. Am logged in (same account) via both Firefox (latest 82.0.3) and my DVD player which can stream YouTube. "Watch later" list shows the same items on both, and I can add/remove on the list via both. I have a playlist of over 100 music videos, it's accessible on both devices.

I have uBlock Origin on Firefox, never see any ads on YouTube. uBlock of course can't be installed on the DVD player so ads run on it.

I'll try clearing the cache - that might help. If not, then I think what I need to do is remove some YouTube cookies, but I'm not sure which ones.

I'll post again after I clear the cache.

Thanks!

If you're not including cookies when you clear cache/history, and you just want to clear the relative cookies, I think those would be cookies set for both google(dot)com and youtube(dot)com.

Good luck.

BTW, personally I tend to think of cache as temporary internet files and usually cookies. History, I tend to think of as just a list of visited sites. That's just me though and I know it's not that simple. :)

I removed a bunch of cookies, and I still get this message when I try to sign in:

<405. That’s an error.

The request method GET is inappropriate for the URL /youtubei/v1/guide. That’s all we know.>

Maybe I need to delete more cookies. I did not delete cache, as such - just cookies, and I selected which ones I wanted to keep.

I seem to be partially signed in, but I am unable to make comments, and I cannot add videos to the lists that I want to save for watching later.

Maybe I should just not make any more comments on YouTube, since they are giving me a hard time about it. I can make them when I use Chrome, but not Firefox. I think it is probably a Firefox problem, or maybe YouTube prefers for me to use Chrome.

That almost sounds like a Google problem but I don't know.

Not knowing and doing a search shows others having that or a similar problem.

https://support.google.com/youtube/thread/46435725?hl=en

I see a suggestion to reset the browser but for me, that would be a last resort. :)

I think Youtube would prefer you use Chrome too. I did for a period of time because I was losing connection in the middle of large uploads in Firefox but not Chrome. I don't remember the exact problem, it's been a long time. But at some point the problem went away.

@Lars, what version of Firefox (you didn't yet mention that)? The link above about YouTube and 405 errors dates to 5 months ago. There have been Firefox updates since then.

I never clear my Firefox cache. History, I usually clear older than 2 or 3 months.

I'm using Firefox 82.0.3 which was updated Nov 9, 2020. The problem started around then, or maybe it started on Nov 9. I guess there is no going back to 82.0.2 to find out😞

I'm trying to follow these instructions, but I think they are not detailed enough:

<reset your browser settings then
reset your cookies on youtube and then sign out and remove account then
sign back in and you should be good>

I am unable to sign out from my YouTube account - it says that I am signed in through my Google account, and I don't think I want to sign out from that, although that might be the only way to fix this. The "Help" sections says to click on my ID photo in the top right corner and then select "Sign Out," but that option does now show up.

As it is now, I cannot upload anything to YouTube on Firefox, and so I am stuck using it on Chrome for now. It's a bit of a pain having to switch back and forth with browsers, but it just means that I will use YouTube much less.

According to Wiki Help, you cannot sign out of YouTube without signing out of all Google related apps:

<It’s not possible to sign out of YouTube for Android without signing out of all other Google apps connected to your account.
>

That would probably work, but I have hundreds of bookmarks that I want to keep, and so it would not be worth it. I store cryptic information in my bookmarks as well in the keywords, and I do not want to lose that.

I have Firefox on my laptop, and I've not had this problem there - so far. I do not have my Firefox accounts linked. It seems that too much linking causes problems.

If you don't mind getting a little techie, you might locate your active Firefox profile (go though Help -> Troubleshooting Information), exit Firefox (important to exit), move the profile folder some place safe such as your "Desktop" and re-enter Firefox. It will create a new profile folder that will probably solve your problem.

You will lose extensions, bookmarks and their keywords (I've did that before too). Like a fresh start.

But since you saved a copy of your profile, you can always retrieve bookmarks, etc.

Profiles - Where Firefox stores your bookmarks, passwords and other user data

I know that's not for everyone but it's really pretty easy.

Worse come to worse you can always re-enter the newly created profile folder, quit Firefox (again, important to exit or some of your changes may be overwritten), select and delete all the files/folders and COPY all the files/folders from your saved profile in there and you'll be right back where you started.

Like I said, it's not something everyone would want to do but... that's probably what I'd do. :)

In fact, I'm on my bedroom computer right now. I have a little one line script that copies my places.sqlite bookmarks file from my living room computer to this computer. That way I have the same bookmarks in here as in there.

I haven't tried Firefox sync in ages. It might be fine now but if I remember correctly, it sorta jumbled bookmarks, putting duplicates in subfolders, something like that. I don't exactly remember. But probably for that reason, I just started syncing relevant files and it's worked fine for me. I don't sync to my phone though. I suppose that's where Firefox sync might come in handy?

ETA: When you go "into" your active profile through Help->Troubleshooting, you'll be inside the folder you want to move. You'll want to go up a level to do the actual move. I'm sure you'd notice that but someone else might not.

An existing Firefox user profile doesn't need to be deleted or lost to create a new one. Multiple profiles can exist and be used with one installation of the program.

Create a separate shortcut on the desktop, Start Menu, wherever desired, for each profile. Edit the properties of the shortcut so the command line references the desired profile.

There are some other command line parameters that may be useful.

Firefox command line parameters

-P <profile-name> -no-remote allows running multiple simultaneous instances of Firefox with different user profiles.

-ProfileManager forces it to ask which profile when opening. And also provides functions to Create, Rename, and Delete profiles.

Bookmarks can be exported to a file, then imported into another profile. I've not done that but it should be possible. There probably are plug-ins to do that which may have more utility than the native process.