This bug is actually a duplicate for #267254. There, the original reporter doesn't care enough to reopen it after enough info was supplied. So, I open a new bug and re-supply all the info, because my system is a bit different.
Firefox and Thunderbird don't have their icons in the taskbar, but use a default X icon instead. Screenshot attached.
Created attachment 207273 [details]
Snapshot with the wrong icon
Created attachment 207275 [details]
Created attachment 207276 [details]
emerge -qpv mozilla-firefox mozilla-thunderbird
Created attachment 207277 [details]
Created attachment 207278 [details]
equery files mozilla-firefox mozilla-thunderbird
*** This bug has been marked as a duplicate of bug 288312 ***