how can i isolate Firefox add-ons singly to find the one at fault- rather than remove them?

No need to remove, there're buttons to disable in order to test.
https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons
https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

List of addons causing issues.
http://kb.mozillazine.org/Problematic_extensions

Disable them ALL and run the browser, then add them in one by one til you get an issue.

I'm currently no longer in safe mode and able to use Firefox.
A number of possible reasons:
  Resetting preferences to default
  (had found there was enough time before crashes to following disable add-ons)
AddThis 3.5.7
bookmarks-deiconiser 2.3
FoxyProxy Standard 3.6.2
Global Menu Bar Integration 3.2.3
Novell Moonlight 3.99.0.3
Test Pilot 1.2.1
Ubuntu Firefox Modifications 2.0.3

I appreciate the comments from you Sam, and particularly the troubleshoot page suggestion 'Turn off hardware acceleration' which i have just done additionally- although will check again when i've gone through the problematic ext. check.
Thanks.

I got rid of:
x 3 https add-ons
x2 Ad bloc add-ons
x2 torbutton add-ons, prior to any success, and will retrace my steps eventually (recording it here).

 I have bought a graphics card (i've not been using one, due to arrive soon) and will be hoping that this will add worthiness to the acceleration fix also, in determining the error more exactly.

Something went wrong when i restarted today, and the crash took place again. I've checked all the exts and the problem appears to lie with Xmarks.

Not that it is because it is enabled directly, but with the inital synchronisation, if as it requests I start the set up process.