what version of firefox works with ubuntu 9.1?

Considering that Ubuntu 9.10 is only supported until the end of this month, I recommend you upgrade to Ubuntu 10.04, which is supported for another year. (If you want, you can then upgrade from 10.04 to 10.10, and around the end of this month, 11.04 will be released.) Then if you still have the issue after upgrading, you could post again.

But 9.10 is still supported, for now, and if you want to try to figure this out for your existing 9.10 system, I'll try to help. Ubuntu 9.10 does have Firefox 3.6.16 in its normal software sources, so you shouldn't have a problem installing it. In fact, on a normal Ubuntu 9.10 system, it would be automatically upgraded to.

So, if you want to continue investigating this issue for your Ubuntu 9.10 system rather than upgrading first, please open a Terminal window (Ctrl+Al+T or Applications > Accessories > Terminal) and run this command, preferably by copying it from your web browser / email and pasting it into the Terminal (and then pressing enter):

apt-cache policy firefox\*

Then select all the text in the Terminal (Edit > Select All), copy it to the clipboard (Edit > Copy), and paste it here.

I suspect that the problem is that the package "firefox-3.5" is installed, rather than the package "firefox", but whether or not that is the case, the output of that command will likely reveal the precise cause of the problem.