I'd noticed firefox running a little slow..
So, remembering that a friend had mentioned to me a week or two ago that it had a nasty memory leak or 16, I took a look at top..
It had only 40mb resident in memory - but the total memory usage was almost 700Mb!
Closed firefox - which took about 60 seconds, with furious mad disk activity - I can only assume writing history and cache to disk. *woomph* 300mb of physical RAM free, swap usage dropped ~500mb (don't ask me how that adds up to just under 700mb, I really don't understand...)
Restarted it - within seconds it was back at a total size of over 200mb. It's now climbed up to almost 350mb - only 57mb of which is resident, thankfully.