Booglebox
Fantastic !! I wasn't getting anywhere with the Task Manager suggestion - got to Task Manager and didn't know what to do - seemed to say that nothing was still going, but FF still demanded a re-start.
I was just about to throw it all away and go back to Chrome, when the suggestion try the Version 28 option - see above - came along, followed by your LTS suggestion.
'course I couldn't download because halfway along the message came back, close down or re-start etc. so I ditched FF and went back to Chrome to download FF LTS, that (seems ? ) to have worked. 24 hrs so far with no repetition, whereas all last week it was happening everytime I tried to open another site.
The old story -WTF do "they" have to change everything ?
Many thanks, hopefully that's solved my problem - for now !!