It is probably the website that includes this number in the page title to notify you and in this case it isn't possible to do anything about it. It is not likely that an extension causes this, but you can easily check this in Firefox Safe Mode.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Does FB have a setting to not notify you in that way? If not...

I think you would need to use either a dedicated title renaming extension (such as https://addons.mozilla.org/firefox/addon/rename-tab-title/) or a userscript for a *monkey extension (such as https://greasyfork.org/en/scripts/30416-title-notification-remover) to repeatedly clean the title as FB modifies it.