Firefox Portable + Xmarks? Problem

I've recently begun using the PortableApps suite, mostly Firefox Portable, at work. I love the flexibility it offers.

I have had a problem where every few days Firefox Portable will not load. The splash screen pops up and the light on my USB drive flickers for a few seconds, but then nothing else happens. It seems to happen at random, sometimes more than once per day or sometimes every few days, and it occurs with Firefox 3.5.2 and also with 3.0.13. When it occurs I have to remove Firefox Portable and reinstall the program. I haven't had any problem with Pidgin, Pnotes, or OO.org, although I don't use them nearly as much as Firefox.

The only three addons I put on my system are Adblock Plus, Xmarks, and ForecastFox Enhanced. I've narrowed it down to a problem either with Xmarks or Firefox, and I haven't been able to find any other solutions or explanations. Since I can't open Firefox when the problem occurs, I'm not sure what shows up in the error log either.

Does anyone have any advice, solutions, or explanations for this problem, or any other troubleshooting recommendations?