There is Zeeker <https://addons.mozilla.org/en-US/firefox/addon/zeeker/>, but there seems to be no new versions recently, and I cannot find a link to the browser add-on on their web site, so they might have abandoned it.

There was also “Dissenter”, still available at <https://github.com/gab-ai-inc/gab-dissenter-extension/releas...>, but the website seems to have pivoted to something else, and the add-on seems to have been removed from the official add-on repositories (possibly due to negative press coverage: <https://archive.fo/sWxAS>). Further discussion: <https://discourse.mozilla.org/t/the-removal-of-the-dissenter...> and <https://www.reddit.com/r/browsers/comments/ptaau2/what_happe...>