Is this with all websites/pages, some, a few?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

It appears to be any page/website. Once it start to slow I don't try to go anywhere else - I just quit. Even force quit doesn't get a rapid response to bring up the finder window.

I did try restarting in safe mode previously, it seemed not to make a difference. I'll try again.

Okay! Thanks FredMcD - since I restarted in safe mode this time there hasn't been the same issue. So perhaps previously I clicked "Refresh" rather than "Open" (which is what I clicked this time....it's not clear why there are two options and what the options mean).

So what next? Presumably I don't start in safe mode every time? HamishMcK

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

I have no addons/extensions listed - there's only "Recommended Extensions" listed.

I've disabled hardware acceleration so I'll see if that makes a difference. Thanks again h

Did you try Scanning your System for any viruses?? or may be try clearing cookies for once. Sometimes these tend to lead to such problems too

What if I uninstall and reinstall Firefox? That's likely to solve it if there's something going on with Firefox itself isn't it? If I do that can I export the history and bookmarks and import them once reinstalled?

That would help only if there is a problem with the program files.

How do I determine if there's a problem with the program files?

I've discovered that if I log into my computer as a different user, Firefox has no problems - does that help shed any light on a possible cause?

Then the program files are okay.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.