try these suggestions
Problems printing web pages

troubleshoot printing problems in Firefox.

try clearing all the cache and stuff on your browser, Tools> clear recent history> check mark those to clear> at the top where it ask for time period use the drop down arrow and select everything> ok

Also clear your printer itself of all print jobs often there is a stuck print job that can cause problems.

some people have had a similar problem and it was zone alarm do you happen to have it as your firewall?

Make sure you have the most up to date version of flash on all your browsers too while you are doing this. You can use the secunia scan to find out what updates you might need.
Secunia Online Software Inspector (OSI)

RC: You know, I have not cared for this 12.0 ever since I upgraded. In addition to the print problem, everything is slower, including downloading, scrolling, etc. Also, things have been moved around, and I just don't like it.

Is it possible to go back to FF 11 (although I don't actually know if that is the last version I was using)? Is it possible for a computer dummy like me to do it? I had no problems whatsoever before.

I am running 12.0 with none of the issues you mention!

Did you clear things out as ravencaujun suggested?

Well I didn't think I needed to clear the browser cache, but I did it anyway, and printing problem seems to be gone!

Thanks RC and LuAnn!

well good for you!

in answer to the question can you go back to an earlier version yes you sure can and there are stored files of the earlier versions should someone need to do that.
You can find old versions of lots of software at oldversions
OLD VERSION

Just keep in mind most things are upgraded for security reasons so going to an older version may affect your security.