I take it your start page is 'open tabs from last session?'

You may have corrupt sessionstore file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above files. Then rename or delete it. Restart Firefox.

I don't know what you mean by delete sessionstore files. I did do the other but after Profile Folder it says Show in Finder and nothing came of that.

I pin my favorite Tabs in Firefox so that I have easy access to the sites I use most. When I restart after shutting my computer down (every few weeks), half the time Firefox opens without them and I have to start all over (and try to remember what some of them were).

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.