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

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

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 file. Then rename or delete it. Restart Firefox.

FredMcD said

You may have corrupt sessionstore file(s). Delete all sessionstore* files and the sessionstore-backups folder. Type about:support<enter> in the address bar. 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 file. Then rename or delete it. Restart Firefox.

Unfortunately that didn't work...

Does this happen when you start Firefox?

Are you using any extensions that deal with Session Restore?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox may be using files in the sessionstore-backups folder as a fallback in cases where the sessionstore.js file in the main profile folder is corrupted and may end up with restoring an upgrade.js backup as the last resort.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

cor-el said

Does this happen when you start Firefox? Are you using any extensions that deal with Session Restore?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox may be using files in the sessionstore-backups folder as a fallback in cases where the sessionstore.js file in the main profile folder is corrupted and may end up with restoring an upgrade.js backup as the last resort. Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

I've tried that but... Nothing changed.

I think i'll go ahead with a new re-install. Two days ago i installed https://addons.mozilla.org/it/firefox/addon/new-tab-override/ but without seeing anything working how it should do, so after a while i've uninstalled it but maybe something keeps to interfere...

For the New Tab Override add-on, I’m not aware of issues with regard to Session Restore.

Rather than doing a re-install, which will reuse your current profile and probably be to no avail, you may be better off if you create a new profile using the Profile Manager (or about:config about:profiles - "Create a New Profile" button) and make sure to run that profile as default, then recover or copy information from the old profile afterwards, as explained in that article.

Since you appear to run Firefox Beta, I’d suggest to do that anyway and not share the current profile with it. More info about creating a "dedicated" profile (whether or not for Nightly or Beta) can be found in the Using a dedicated profile for Firefox Nightly article.

Side note: in case you skipped Firefox 55 and shared the current profile back and forth between 56 beta and 54, chances are you are suffering from a migration in some profile files as of 55. Some warnings for not sharing profiles between 54 and 55(+) - or repairing it by performing a Refresh when reverting to 54 - were issued. From the head, some files are now compressed.

_{/Edit: of course, about:profiles - thanks :)}

^Quote: (or about:config - "Create a New Profile" button)

That would likely be the about:profiles page: (or about:profiles - "Create a New Profile" button)