That feature is still in Firefox, it checks "compatibility" as the update starts, and gives you the opportunity to stop the update if you choose to do so.

Try this extension.
https://addons.mozilla.org/en-US/firefox/addon/is-it-compatible/

Apparently I wasn't clear or the word upgrade wasn't right. I meant going from 4 to 5. I downloaded it, installed it and got no option resembling that at all, it just saved a DMG to my hard drive and then told me to put it in my applications folder as always. I opened it and that was that, no compatibility check. I already got one error about an add-on that's vital. I'd backed up before I installed and saved Firefox 4 as a separate file before opening 5, I hope I'll be able to go back to it. The old system was really much better, what's the point of finding out after you already installed something if your systems will work? Or of avoiding installing because you're afraid they might not when everything very well might be fine? I can guarantee you that I won't be installing an major updates for a very long time, that's for damn sure.

Downloading and installing a new version of Firefox is different than doing an actual "upgrade", like when Mozilla notifies the user than an upgrade is a available and the user says "ok".

I get that now, I'm glad I backed up. But compatibility is still just as big an issue and there's no way to check before installing. So please consider this my plea for a better system, especially if what I read about a faster cycle of new versions is true. Thanks for your help.