If you look at Firefox Add-ons under Tools, it indicates that particular plug-in is not compatible with Firefox 3.5.2.

Thanks for information & links I have used your information at another forum to alert those users as well, I could see this turning into a nightmare for some users.. Thanks, zep

When I have uninstalled firefox before installing a newer version the bookmarks, history, etc. have stayed in the Mozilla folder. To be double safe you can export your bookmarks (bookmarks > organize bookmarks > import and backup) to a file in your My Documents folder before uninstalling. That way you can delete the folder after uninstalling if you need to do so to install an older version.

you should not lose anything it is just an update version you should be able to just go to the update button on your current firefox and check for updates then select to allow it to update, you will have to shut down and restart firefox once it is done, there are some differences but not that major and there are ways to change the things to look like they did in 2.0 there is a thread on here somewhere that I put a lot of links on to make those changes. (you can search for it) they are ending support for version 2 in the very near future (weeks) so everyone should begin to read up on it and plan on the update. there are a few add ons that are not yet compatible with 3 but 3 has been out so long now that many have already become compatible. I always back up at least my favorites file before doing a version update and sometimes my profile also, to do your favs back up just go to bookmarks, organize bookmarks, click on export, it will open up your storage area on your pc simple save the bookmarks file to some place on your pc or on your desktop where you will remember where it is, that way should anything go wrong you have your bookmarks file right there saved to put into the new version. That is why I really like the add-on foxmarks, it saves all your bookmarks to online, you have a free account, you can log in to it from any where and have access to all your favs, and if you should ever have something happen to your firefox everything is still right there on your foxmarks account and can be synced right back to your browser, and every time you add a new bookmark foxmarks syncs up with the online one and puts them there also. Profiles How to Back up Mozilla Firefox and Thunderbird this is an older article but shows some of the differences Review of some changes in Firefox 3 one of the big differences is the URL bar on 3 if you don't like it you can change that too Firefox 3: Make Address Bar Behave Like Firefox 2