In both mozilla-firefox-bin and mozilla-firefox, the back button does not function correctly. After a new page is loaded, clicking the back button causes the tab label to say "Loading..." and the spinner in the upper right corner to spin. However, the status bar still says Done and the browser does not go back. Clicking the back button a second time causes the page to actually go back. Pushing the back button when not at the end of the history however does not have this behavior. When not at the end of the back/forward history, all works as expected.
FWIW, I tried the binaries from mozilla.com and I have the same problem. So, I guess this should be reported to Mozilla? Or could it still be a Gentoo issue?
Upstream issue, they are aware of problem and I am tracking it.