Do you mean pop-ups telling you there is a problem with your computer or your Firefox and you need to call an 800 number and pay for tech support? Annoying scammers... If you have one now, try pressing the Esc key quickly several times in a row. If it keeps coming back, look for a checkbox near the lower right of the popup offering the option to stop showing more dialogs, check that, then click OK. Those often are spread by deceptive ads, so you could consider using an ad blocking extension such as: https://addons.mozilla.org/firefox/addon/ublock-origin/ (on some sites, you may need to use its toolbar button to make an exception in order to see all the content). Finally, if you want to keep the page open but do NOT want their audio, look for a little speaker icon on the tab. It's a mute/unmute button. See: [[Mute sound in Firefox tabs]]. If you prefer silence in general, there is an add-on to set Firefox to mute by default, and then click the speaker icon when you want to hear the audio: https://addons.mozilla.org/firefox/addon/auto-mute/