Vastest majority of the instability is some combination of extensions, plugins and user error. Same applies to memory leaks, barring the page caching "issue", which was an oft-requested feature (and how to turn it off's been well enough documented).
Gives the Opera fans something to shout about though, dull though all this browser fanboyism is.
Considering I'm running nightlies (and have been for a while now), Firefox has overall been pretty stable, and I've never seen bookmark loss either (even punting them over to Places was painless). Certainly nowhere near "a lot", even with a
major crasher currently unpatched.
Not that it'll make an ounce of difference, for comparison closed both Opera and FF, and opened the same tabs in both. Firefox uses 58MB/50MB VM, Opera 48MB/45MB VM, so Opera's hardly lightweight (especially given I've got 15 extensions installed in Firefox).
For reference, whatever's happened with the Firefox profile is a complete oddity, and I can't say I've
ever heard of it before.
To the OP: wouldn't mind a zip of %APPDATA%\Mozilla\Firefox\ if you could, email in trust.