Sometimes after a relaunch Firefox refuses to load anything - addons don't start, web pages load eternally instead of instantly.
Sometimes on launching Firefox, my system gets into a state where nothing seems to work. (This has been going on for years, but only intermittently) My addons stop working (for example, clicking on the icon for LastPass just pops up a small box with nothing in it) and pages never load beyond a blank page. Sometimes the page that was the active tab before closing Firefox will have a partial load of the page (I've had GMail come up to the "loading GMail" screen with the big envelope, but it never proceeds beyond that) but even that's inconsistent.
I don't use profile sync; I usually just copy my profile around. I've tried recreating a brand new profile and migrating my bookmarks, then adding in addons.
Any thoughts about what could be causing this?