It sounds like you have the "Always show the tab bar" preference unchecked. To change that go to:

1. Click on the Firefox button and then select Options
2. In the Options window, select the "Tabs" panel
3. Check the box next to "Always show the tab bar"

"Tools" | "Options" | "Tabs" "Always show the tab bar" was checked - uncheck it. (Restart Firefox and confirm settings.)

First PC Window 1: [+] at far right - all tabs visible. Window 2: [+] at far left - one tab missing. Window 3: [+] at far left - one tab missing.

Second PC Window 1: [+] at far right - all tabs visible. Window 2: [+] at far left - one tab missing.

Sorry - I don't think it made any difference. There is a pattern here, and I do have "Show my windows and tabs from last time" selected. I think opening an empty Firefox and adding tabs manually is OK.

Try to create a new profile.
That is the only solution that has worked for me as I didn't manage to fix it in the profile and I couldn't identify the cause.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If the new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

You can also try to reset Firefox:

• https://support.mozilla.org/kb/Basic+Troubleshooting

This is beginning to sound like some of the anti-depressants I've taken - the solution is worse than the problem. It's going to need small changes, then open more than one window with more than one tab each, then restart. I'll take it slowly.