Ublock origin
Best adblocker by a huge margin. Requires no setup and just works really well.

Umatrix
Kind of a more advanced version of ublock origin. Requires quite a bit more manual effort to get working nicely.

Decentraleyes
This one packages a lot of common libraries and makes your browser locally grab them instead of connecting to google or cloudflares servers. It improves performance and security and requires no setup.

SmartHTTPS
Makes sites connect with https which encrypts your traffic to the site if any site supports it, but doesn't automatically redirect you. If you're currently using httpseverywhere you should switch because httpseverywhere just uses a massive whitelist which is silly and basically completely ineffective. SmartHTTPS works by always requesting the https version of any sites you connect to and falling back to http if it doesn't support it.

TabStore
This one lets you save some or all of your currently open list of tabs to a plain text file. Later you can load and restore those tabs from that file and of course you can just open and edit the file in any text editor. You can send that file to someone or do whatever you want with it. If you use youtube-dl you can even pass a file full of youtube/soundcloud/whatever links to download all those videos. I wrote this extension because I couldn't find anything that did this. Literally all the other existing extensions to save a list of tabs would either upload it to a weird website or require an account or generally do weird stuff. Source code is here if any of your are interested.

NewTabOverride
I passionately hate the new tab firefox provides. This extension lets you set your newtab to whatever you want. Like a search engine or something.