UPDATE 1.1:
There is now an options page available for the add-on! It allows users to add keywords they wish to be blocked in their URL.
I was very unsatisfied with how I had hard-coded the keywords in the previous version, but now users can add whatever words they like to their own list. The list is saved via the browser storage, and a list of the already saved keywords is available in the options page.

NOTE: If the list doesn't update after pressing the "add" button, try again and / or wait a few moments. You should see the list change within a few seconds.

I will deal with how ugly this extension is in a future update.