Steps

Please, take a look at the open pull Requests and make sure that your changes haven't been already proposed.
Communication is key: Before investing your time on any changes or implementation, start a discussion describing your idea and implementation details. We want to avoid anyone being hurt because they felt they "wasted their time" implementing a rejected feature.
Once you have implemented your changes:
1. Run the JavaScript linter (Node.js is required):
```
npx jshint *.js options/*.js
```
2. Run the CSS linter:
```
npx stylelint options/*.css
```
3. Lint the entire extension:
```
npx web-ext lint
```
4. Format the code:
```
npx prettier . --write
```
Test the extension thoroughly on both desktop and Android.
Finally, open a pull request. Congratulations, you made it! 🥳

Useful links