Look for “Profile Directory” in the page. Click on “Open Directory”, and create “'chrome” folder inside, if there isn't one already. Now, the top tabs bar should be hidden. Optionally, hide the titlebar to reclaim some pixels, note that this will hide your window buttons.

Look for “Profile Directory” in the page.

don't know where this is,

edit:

if you mean this directory C:\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\<somthing>.default-release

then as you said i created a "chrome" folder but the blue bar at top is still there.

Yes, I would be amazed if that made any difference. What add-ons do you have? Is the line still there if you start in Troubleshoot Mode?

https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems 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 (also deals with hardware acceleration)

Are you using a multiple monitor setup ?

Is the line still there if you start in Troubleshoot Mode?

yes its still there, even when freshly installed.

Are you using a multiple monitor setup ?

No i have a single monitor.

You could try a new profile. If that doesn't work, you could file a bug report.

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles#w_start-the-profile-manager-when-firefox-is-closed https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_what-information-is-stored-in-my-profile https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://bugzilla.mozilla.org/ https://support.mozilla.org/en-US/kb/file-bug-report-or-feature-request-mozilla