I installed ubuntu's Edgy-Eft distribution early this month. I'm very happy with it. But... the Google toolbar wouldn't install in the firefox version that came with the distribution. Firefox would tell me that it wanted plugins compatible with linux-gnu_x86-gcc3.
Happily, it's easy to fix. First, you download the .xpi file. Despite the extension, it's in zip format. Unzip it, and open the file install.rdf and look for where it says
and replace it with
This package is signed, so you also have to delete the signatures for firefox to take it. Just delete the directory called META-INF to erase the signatures.
Finally, zip the contents back into your .xpi file, and point your browser at it. It should install without a problem.
Of course, I disclaim all liability for anything that might happen if there are problems... and while I don't thing the nice people at Google will have a problem with you doing this, I expect they'll also prefer that you keep the modified package to yourself.
I was able to get the Mozex plugin working using a similar method (tell it that it is, in fact, compatible with firefox 2.0.*).