Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

FredMcD said

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

I preemptively answered this.

None of the pages you referenced share the symptoms described above.

Please do not be so dismissive of the information in the original post.

@Steve none is being dismissive there has to be diagnosis steps to figure where problems are starting from. If you really want help making disparaging remarks towards those helping won't get much help.

You can check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page
  in case errors are reported close and restart Firefox and retry

See also:

• http://kb.mozillazine.org/Locked_or_damaged_places.sqlite
• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

You can check for problems with the sessionstore file(s) in the Firefox profile folder that stores session data.

• delete sessionstore.jsonlz4 and sessionstore.js

You can also check for issues with the files in the sessionstore-backups folder.

When sessionstore.jsonlz4 is deleted you lose pinned tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

cor-el, thank you for the thoughtful recommendations on this matter though they didn't directly fix the problem. I did take your idea that some file in my Firefox profile may be the culprit. So, I took out the small sledge hammer and did a Firefox refresh (big sledge would be a clean install). Unfortunately that meant spending a bit of time getting Firefox set back up the way I like it, but it has solved the problem and restored the usual fast response times to which I'm accustomed.