Consequently, I have returned to using Opera, although it's decidedly better than the Opera I used to use back in the day.
Unexpectedly, and I have no idea what it's doing right (or Firefox is doing wrong), but not only is my PC running a lot more smoothly due to the reduced memory load, but webpages are loading noticeably faster. I'm currently shaped (stupid lack of bandwidth), so had just accepted that pages would naturally load at a snail's pace - but Opera is somehow getting them to me in pretty good time1.
The downside is that I had Firefox pretty well tweaked to work the way I liked, and Opera seems not to be quite as customisable - but I think it's partly this high level of customisability that was bringing Firefox down.
So, yeah, I think I'm better off with a faster, more resource-efficient browser than one that I've got set to hide the number of messages in my gmail spam folder.
1. Okay, it's still a snail's pace, but it's noticeably faster than the "start loading a page, go and make a cup of tea and maybe it will be ready when you come back" I've been getting with Firefox.