Here's a dumb problem I caused for myself. I noticed that shockwave flash/whatever videos weren't playing for me in Firefox, after getting a new internet connection after about a month of nothing (hello, 919 email messages, almost entirely spam).
So I spent a few hours trying to fix it, closing and reopening firefox between deletes and reinstalls of plugins, to no avail.
Then I noticed I had "Flash Click to View", some kind of style extension, installed - presumably without knowing what it was, probably not long before I lost my connection.
So I moved %APPDATA%\Mozilla\Firefox\Profiles\default.mf3\chrome\userContent.css to a backup directory, reopened Firefox and all was well. Try this if you're perplexed and experiencing similar problems (and, like me, are clearly an idiot for installing it without noticing in the first place).