A few months ago, I wrote a Firefox add-on for e621:
https://e621.net/forum/show/87369

It's hosted on Add-on Builder, but that site has gone into read-only mode and will shut down in April 2014.

I can't submit the add-on to AMO, because:

https://addons.mozilla.org/en-US/developers/docs/policies/reviews
Prohibited Add-ons
- Add-ons whose main purpose is to facilitate access to pornographic material, or include references to this material in their descriptions

What would be the best place to host e621-related browser add-ons?
They're basically zipped text files, and mine are less than 20 kb in size.

Make your addon work on e926.net, and in the screenshots and documentation make references to e926 instead of e621.

It can still work on e621, and you can probably still mention that, but as long as you don't present e621 as the main site it should be used on, you're complying with the AMO rules.

Use Google Drive, make a Wiki for your Add-On and link to it there