Firefox 55.0.3 started using >100% CPU.(Mac)

[8-bit]

So, I did a FF refresh, it re-imported my bookmarks etc, and I then reinstalled 6 extensions so far, and have a hundred tabs+ opened. FF is at ~10% CPU.

Does that mean something was corrupted, and does it indicate what that might have been?

Appreciate the responses, BTW!

You originally stated that you had 12 extensions, and so far you have re-installed 6 of them (all seems good). There is a good chance that one (or more) of the remaining 6 extensions was causing problems. If you plan to re-install those other 6 extensions do so one at a time and then test. This will help you isolate things (although you did do a FF refresh, so that is part of the mix as well).

[8-bit]

BTW, I run FF on my laptop using "portable Firefox". So I deleted that app and reloaded with a version on my back up from a month ago and it seems to be running OK...

What version of Firefox did you re-install from your back-up? The reason I ask is that if you are going to do a fresh install on the iMac you probably want both versions of Firefox to be the same.

Moving forward (below), I am assuming you are going to do a fresh install with the latest release version (56.0.2)

I do want to move forward with installing a fresh version on the iMac and be able to synch with the laptop

Here is all you have to do to have a brand new fresh install:

[0] Open your current version of Firefox and export your bookmarks to the Desktop (precautionary measure)
[1] Download the latest stable release to your Desktop
[2] Go into the Applications folder and move Firefox.app to the Trash
[3] Go to ~/Library/Application Support/ and move BOTH the Firefox folder AND the Mozilla folder to the trash
[4] Go to ~/Library/Caches/ and move the Firefox folder to the Trash
[5] Go to ~/Library/Preferences/ and move org.mozilla.firefox.plist to the Trash
Everything on your Mac related to Firefox is now in the Trash. ***** DO NOT EMPTY THE TRASH UNTIL EVERYTHING IS RUNNING WELL *****
[6] Open the latest stable release .dmg on your Desktop (from step 1) and install Firefox as directed.
[7] Open Firefox, set your prefs, and re-import your bookmarks from the Desktop

You now have a TOTALLY CLEAN install.