Guys, thanks for the posts.
I tried Avast some time back because I heard good stuff about it but couldn't stand the voice. Presumably there was a way of turning it off but I think turning it off turned off other sounds as well (fuzzy memory here).
I'll try the free-av and outpost, as you recommend and post back.
Incidentally, I've re-installed AVG and the stickiness seems to have abated. Then again, I only installed the bits I wanted this time so maybe there was something in the bits I didn't use that was affecting Firefox.
I've been putting off upgrading Firefox because I have some add-ons I really like that aren't compatible. But that's the trouble with add-ons, I guess. Give me time, it'll happen!
Thanks again.