This issue can be caused by corrupted cookies.

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > Cookies: "Show Cookies"

tried it, and this doesn't help.

still getting message "Sorry, Bad Request. Your browser sent a request that this server could not understand.

Size of a request header field exceeds server limit."

and the cookie that message shows is not in my cookie list.

also, your reply doesn't answer the main question - when will firefox offer an upgrade for mac ox 10.4.11 users? the sites i'm having problems with say i need to upgrade to firefox 4, but firefox 4 doesn't support os 10.4.11

not cool!

If clearing the cookies doesn't help then it is possible that the file cookies.sqlite that stores the cookies is corrupted.

Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete cookies.sqlite-journal and cookies.txt, if they exist, in the Firefox Profile Folder in case the file cookies.sqlite got corrupted.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

There won't be any new Firefox versions released for Mac OS X 10.4 and especially not for Macs with a PowerPC processor.
The Firefox 3.6.x versions are the last Firefox versions that will run on PPC and there won't be many more security updates.

For an unofficial Firefox 4 compatible version (TenFourFox) that runs on PowerPC Macs with OS X 10.4.11 or OS X 10.5.8 you can look at:

• http://www.floodgap.com/software/tenfourfox/