I like finding little gems, here's a few I've found.. share yours please.
Extension source viewer, a MUST have for anyone installing addons, view or download the addon code to review for dubious code before installing... https://addons.mozilla.org/en-US/firefox/addon/crxviewer/
Open bookmark in Container Tab, works as title says... https://addons.mozilla.org/en-US/firefox/addon/open-bookmark-in-container-tab/ ...
Containerise, easy to set sites to open in specified container... https://addons.mozilla.org/en-US/firefox/addon/containerise/
nextpage, works from first page on Reddit and other sites but some it doesn't until you go to the second page, after that it works well... https://addons.mozilla.org/en-US/firefox/addon/nextpage/