I recently cleaned FF anyway but should also have mentioned it used to happen with previous versions though not to this extent
anyway in case others happen on this, I found this thread [forums.mozillazine.org] - there is some advice in there (#2928065) about read only files - specifically sessionstore.js and cookies-nn.txt - which could cause the problem
In reading it seems it's a bug, but I only had one sessionstore.js file and loads of cookies-xx.txt files.. I followed the advice, cleared the cache, exited FF and deleted all the excess files before restarting. There was no difference initially but then going on the line that there are problems if the files are 'read-only' I also made sure (made a copy of default profile first) that the profile folder (and all the child files) were not read only.. not sure if that will do any harm?
I have had a couple of 'unexpected' shutdowns lately, so I'm presuming that one or other of the files that was supposed to writeable was not and was locking?
anyhow it seems to have worked - all is now going superfast again, I'm not sure which file in the profile was causing this I only had 1 sessionstore.js and about 12 cookies files, but anyway.. for now all seems good again