Screenshot of the issue? If you installed 3d party apps or userChrome.css you need to disable those software to insure this isn't apps caused issue or not. Did you try Firefox Private Mode browsing to also check for issues if Addon caused or not.

Thanks anyway.

These are the possible values for the backspace button in about:config. browser.backspace_action

// Backspace and Shift+Backspace behavior // 0 goes Back/Forward // 1 act like PgUp/PgDown // 2 and other values, nothing

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

You can hold down the left mouse on either Back/Forward toolbar button to open the tab session history. It is possible that there are multiple entries created by JavaScript and in that case you may have to select one entry manually.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.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 Troubleshoot Mode start window

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