What does the message look like? I've attached an example from a site where I started typing a reply and decided not to finish, and I get a warning when I try to close the tab. Is it that message? But it's popping up frequently/inappropriately on sites that want to trap you? The reason it would be helpful to get more context is to try to assess whether it's just a particularly annoying site or whether you might have malware or a bad extension installed. "Just in case:" <p>Here's my suggested procedure for tracking down and cleaning up bad add-ons and other hijackers. I know it seems long, but it's really not that bad. (If you do not use Windows you can skip to #2.)</p> <p>(1) Open the Windows <strong>Control Panel</strong>, Uninstall a Program.</p> <p>After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. <em>Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it.</em></p> <p>Take out as much trash as possible here. If you're not sure, feel free to post program names or a screenshot of the list.</p> <p>(2) Open Firefox's <strong>Add-ons page</strong> using either:</p> <ul> <li>Ctrl+Shift+a (Mac: Command+Shift+a)</li> <li>"3-bar" menu button (or Tools menu) &gt; Add-ons</li> <li>in the Windows "Run" dialog, type or paste<br>firefox.exe "about:addons"</li> </ul> <p>In the left column, click <strong>Extensions</strong>. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. <em>All extensions are optional; the troublemaker might not be obvious from its name.</em></p> <p>Often a link will appear above at least one disabled extension to restart Firefox to make the change effective. You can complete your work on the tab and click one of the links as the last step.</p> <p>Any improvement?</p> <p>(3) You can search for remaining issues with the <strong>scanning/cleaning tools</strong> listed in the previously mentioned support article: [[Troubleshoot Firefox issues caused by malware]]. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.</p> Hopefully that will flush anything on your system and in your Firefox that could be a problem.

Sometimes a website will want to confirm that you want to leave that site when you click a link.