(Firefox 57 and above) Click the Library button 57 library icon and choose History.
   (Click the menu button New Fx Menu and choose History in older Firefox versions.)
   In the History menu, choose "Clear Recent History...".
   Under "Time range to clear", select "Everything".
   Now, click the arrow next to Details to toggle the Details list active.
   From the details list, check Cache and Cookies and uncheck everything else.
   Now click the "Clear Now" button. 

but This just temporarily log you out of all sites i logged in to. i need a permanent solution .

when i click my profile picture, there is no link and dropout menu coming for log out. Chosen solution. i follow this step (Firefox 57 and above) Click the Library button 57 library icon and choose History. (Click the menu button New Fx Menu and choose History in older Firefox versions.) In the History menu, choose "Clear Recent History...". Under "Time range to clear", select "Everything". Now, click the arrow next to Details to toggle the Details list active. From the details list, check Cache and Cookies and uncheck everything else. Now click the "Clear Now" button. but This just temporarily log you out of all sites i logged in to. i need a permanent solution .