The set of clueless users who would install arbitrary malicious extensions and the set of clueless users who would download and run arbitrary executables is basically the same. Yet if they do the latter, any extention sandboxing is completely and utterly moot. It's the same issue as with mandatory extension signing; requiring signing for extensions while nothing stops someone from downloading an unsigned .exe (which could memory-patch Firefox if it so wished) is absurd.
When Firefox is no longer an attractive option for power users, the only thing it will have going for it is ideology. And that will not be enough to compete.
I do not consider myself a power user of Firefox, but the two extensions I use (DownThemAll and tab groups) are not going to be ported.
Servo has been in Dev for 5 years, so if google isn't working on something to compete with that, Firefox is going to completely dominate the web for years.