Because Firefox 4 is no longer secure it is recommended that Firefox 3.6 or firefox 5 be used.

If the game is using an add on try installing and running compatability reporter

If it is the games website that is a problem

• contact the games website they may have plans to make it compatible very soon
• consider downgrading to Firefox 3.6 or trying the add on https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/ (unfortunately that add on site requests a money donation ) or using another browser for your gaming.

What is the game and gaming site that is causing a problem ?

I would also recommend that whilst on Firefox 5 you use the Feedback option to provide the URL of the problem site, at least that results in your feedback being drawn to the attention of Mozilla Firefox and its developers.

If you do need to tue Firefox 3.6 consider using the portable version, you can install that easily whilst continuing to use Firefox 5 for ordinary browsing. See http://portableapps.com/apps/internet/firefox_portable/localization#legacy36

That answer doesnt answer the question posed

Unfortunatley Firefox 4 is unsupported and is not secure. The Firefox staff are asking that we do not assist users in using firefox 4 but provide help in finding other solutions. You could look at other sites: http://kb.mozillazine.org/Go_back_to_an_old_version_of_Firefox

If you find a Firefox 4.0 you better hang onto it. The development people seem totally unresponsive to real FF5 bugs and usage problems and released an apparently untested version on users. I say "untested" because many reported problems are immediately apparent on initial startup. Possibly it's just that available test platforms were very limited in HW/SW breadth.

I don't know what to do at this point. Firefox 5 is unusable because I can't use my bookmarks, FF4 is unavailable for reversion and said to be dangerously unsecure (actually it's out there -I found it; look around.)

Possible I'm excessively loyal to FOSS software and Mozilla because I haven't yet started to look around, still hoping for FF5 to magically fix itself.

I have to add that I've been very pleased with previous versions going back to the beginning.

Keeping the original installers, or having a reversion strategy in the OS can help, and as you say Firefox 4 is relatively easy to find.

Firefox 5 generally should be better, and I see no reason Fx5 should give problems with bookmarks, if you start a new thread about that I am sure someone will help.