The above message has indeed become less frequent compared to early versions of Firefox, but certain extensions are prone to break again and again. Particularly the most important one: Pentadactyl.
The Dactyls are strange people since they usually fix the problem within a couple of days, but then forget to change the maxVersion entry in the install.rdf. My usual routine looks then like that: I open a terminal, navigate to the folder containing the pentadactyl-nightly.xpi, and open it using vim (yes, vim can open zip archives directly).
vim pen〈TAB〉〈ENTER〉 /install〈ENTER〉〈ENTER〉 /maxV〈ENTER〉 wwlR 8 /* at least as high as the current version */ 〈ESC〉 ZZ ZZ
Dragging the repaired xpi into Firefox' add-on manager concludes this procedure which never takes me more than 30 s. There are other ways to deal with that issue, but I prefer this one.
PS: Right, I don't use Opera anymore.