docs(README): extension relisted on Mozilla Addons (#106)

* Update readme - addon back in Firefox store

See #94 (comment)

* Update README.md

README.md

@@ -11,10 +11,7 @@ A cross-browser WebExtension that serves as a web browser watcher for [ActivityW
 Install for your browser:
 
  - [Chrome][chrome]
- - ~~Firefox~~
-   - Due to Mozilla incompetency, the addon is no longer listed in the Mozilla Addons store (see [discussion #818][818]).
-   - ~~You can install the latest Mozilla-signed version [here][last-xpi].~~ (we are apparently not allowed to self-distribute this, see discussion linked above)
-   - You can also build from source, see [this comment][build-source-cmt] and instructions below.
+ - [Firefox][firefox]
 
 [activitywatch]: https://github.com/ActivityWatch/activitywatch
 [firefox]: https://addons.mozilla.org/en-US/firefox/addon/aw-watcher-web/