The problem is arising too infrequently now for me to try any real trouble-shooting.

Two possibilities:

A new Firefox version (= effective reinstallation)

Three years after plugging in a mighty 16MB graphics card (stop sniggering at the back, please), I realised that the BIOS had not automatically detected it (ancient Dell GX260.) Manually selecting AGP video controller has a) improve the graphics noticeably - almost plays YouTube full screen now - and seems to make the machine run so much better...