If you’ve never used about:config, you’re missing out. Type about:config into your Firefox address bar and hit Enter. One easy configuration change is limiting animated images. Double-click image.animation_mode and change it to none to stop all animated images, or once to let them run through once. You can revert to the default behavior with normal.
Back to the about:config screen: Firefox has a special “Back-Forward cache” for recently visited pages that works differently than the regular browser cache. The default setting for browser.sessionhistory.max_total_viewers, “-1,” will save up to 8 pages if you have more than 512MB RAM on your PC. Reduce the number to zero for maximum memory savings