Hi, d38dm8nw81k1ng
I'm running Firefox 1.5.0.4 (but not on Gentoo) and all the buttons seem to be working fine. Maybe it's something to do with the way the Gentoo devs setup the package.
The only other thing I can think of is to try deleting your browser cookies and cache to see if it makes a difference. If that doesn't help, you could kill Firefox, rename your hidden .mozilla file in your user directory, then restart Firefox to see if a fresh set of .mozilla files might help.
Let us know...
ozar