You can find them on the Firefox Addon page and search for popup blocker and decide which one you want. You can read the popup blocker reviews and decide which one works for what you want.

https://addons.mozilla.org/en-US/firefox/

Firefox for Windows, macOS, and Linux comes with a built in popup blocker since forever. https://support.mozilla.org/en-US/kb/pop-blocker-settings-exceptions-troubleshooting

There are however some extensions that can add popup blocking features and there are also Ad block related extensions at https://addons.mozilla.org/firefox/