Is this like a pop-up ad? For regular pages, you may encounter these issues: * New window (back button gray): try closing the new window to see whether your original window is behind it * New tab (back button gray): try closing the new tab (Ctrl+w) or click the earlier tab on the tab bar * Back doesn't work: some sites add an extra page to history, so you need to right-click the back button to get a list of history for the tab and skip over the intermediate page * Full screen (no access to toolbar): Firefox's keyboard shortcut for switching between full screen and regular views is the F11 key Ads that use Flash can expand beyond the usual page boundaries without using Firefox's full screen feature. Usually, pressing Esc will return them to normal view. Is any of that helpful? ---- To rule out one of your extensions as a possible culprit, you try the site in Firefox's Safe Mode. That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: [[Troubleshoot Firefox issues using Safe Mode]]. You can restart Firefox in Safe Mode using Help > Restart with Add-ons Disabled ''(Flash and other plugins still run)'' In the dialog, click "Start in Safe Mode" (''not'' Reset) Any difference?