Well, thanks everyone for their prompt and (mostly) patient responses. I have tried each and every method described on here to fix this problem which, thankfully, I was not imagining after all. First, I closed and re-opened Firefox. Didn't work (and thanks for whoever suggested that). Then I restarted my computer. Didn't work. Then I uninstalled and re-downloaded and reinstalled Firefox (latest version). Didn't work. Then I tried doing a system restore; this probelm happened last week, and I restored to three weeks ago. Didn't work. Then I tried re-naming the target as per Keichi's instructions. Didn't work. Finally I located, renamed and imported the bookmarks.bak file into Firefox. Didn't work.
So me and Firefox are through. This is an absolute bull**** problem for a program as big and as popular as firefox to be having. It's never been fixed, as I am always running the latest version of Firefox. All my bookmarks for teachers' college are gone, as are all my school-related research links. I guess I should have backed them up, but so many people haven't even heard of this problem, I can hardly be blamed much for that.
And before some of you start telling me how bad IE is (bearing in mind that I never ONCE said I would ever consider returning to a browser somehow even crappier and more problem-filled than Forefox), I guess I'm going to have to start looking at Opera after all, because Netscape can go suck rocks.