No do not do it. If do any thing do this and do back up your Profile matter of fact copy the whole profile. Too many bookmarks and sessions being lost this way.

Follow this is same thing I guess but want you to backup and export and copy out.

It will continue to nag you then poof it did it.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

Thanks for the response: your suggestion may resolve the problem, I may give it a try in due time. But it raises some questions in my mind:

1) If the process truly preserves the profile (and thus all the extensions, both enabled and disabled), then it would seem to indicate that it is not an extension causing the problem (others have suggested using safe mode, which implies it is an extension problem).

2) If running Windows Disk Cleanup is the solution to the problem, why would it be necessary to uninstall Firefox and reinstall?

3) If uninstall/reinstall Firefox is the solution, why would it be necessary to run Windows Disk Cleanup?

4) If either Windows Disk Cleanup or uninstall/reinstall Firefox doesn't that imply that there is a bug in Firefox upgrades that should be found and fixed? And that this process is simply wiping it out instead?

1) you should do this before it decides to do it self as I warned. When you delete the folders it becomes a nice clean install instead of copy over and left overs of updates and such. Same as a Fresh Install of Windows is better than a Refresh.

2) Running Disk Cleanup is my idea to stick in as more than 3/4 of people have no idea it exists let alone Win10 will have 300-800gigs of updates from the large Creator updates. So this slows the computer, takes ram and space. As is telling people it Should be Pinned and run Weekly.

3) same you repeated.

4)Would you prefer a Refresh and wipe out all your settings..... The reinstall will not do that. If have issues after then can look at the Profile as a issue. look, you asked for support, you got it. Take your assumptions and do nothing or follow what I have posted. Is not my issue if you do not wish to comply with the suggested recommendation.

Thank You For Contacting Mozilla Support.

A while ago, Firefox used to display unresponsive script warnings with the script URL (Warning Unresponsive script - What it means and how to fix it). I don't know whether that information is still available; maybe it's in a console somewhere? If I see the new message on a site, I'll check the consoles, but I haven't seen it yet.

A common problem with the old dialog (in addition to long URLs having ... in the middle) was that it often didn't help to know the specific script that was slowing down Firefox because you couldn't load the page without running it. Unless it was a third party script you could block with an ad blocker, uMatrix, NoScript, etc., in which case one of those add-ons might help.