Useful tricks for Firefox

Some settings for Firefox cannot be adjusted in the normal way (via the “Settings” menu item).

Remove service workers

Open about:debugging#/runtime/this-firefox in the address bar.

Delete all listed service workers by entering

for (let button of document.querySelectorAll(".qa-unregister-button")) { button.click(); }

in the debugger console.

More than 4 rows of top sites on start page

Open about:config in the address bar.
Enter browser.newtabpage.activity-stream.topSitesRows in the search field.
Enter the desired number of rows.

Keep backspace key from going back to previous page

Open about:config in the address bar.
Enter browser.backspace_action in the search field.
Set the value to 2 to unmap the backspace key.

Customise/hide elements on websites

In the address bar, enter about:config.
In the search field, enter toolkit.legacyUserProfileCustomizations.stylesheets.
Set the value to true to enable custom stylesheets
Open the user folder:
Linux: /home/[username]/.mozilla/firefox /[profile-id].default-release/
macOS: /Users/[username]/Library/Application Support/Firefox/Profiles/[profile-id].default-release/
Create a subfolder called ‘chrome’ there (if it does not already exist) and create a file called ‘userContent.css’ within it.

In this file, define styles for elements on specified domains, e.g.

@-moz-document domain(www.ex-parrot.de) {
   .sample-class {
      display: none !important;
   }
   #sample-id {
         font-weight: bold !important;
   }
}

Activate new/modified styles by restarting Firefox.

Add custom search engines (no OpenSearch Support)

Open about:config in the address bar.
Enter browser.urlbar.update2.engineAliasRefresh in the search field and set it to true.
Go to “Settings” → “Search.”
In the “Search Shortcuts” section, you should now see an “Add” button.
Click “Add” and fill in:

Name (e.g., “Custom Search”)
Search URL (replace the search term with %s, e.g., https://www.example.com/search?q=%s)
Optional keyword for quick access

Save your new search engine.