You actually can edit your redirects in Firefox Mobile (at least it works in Nightly). There is a focus bug in the Edit Redirects popup.
Tap the Edit Reditects button. Nothing seems to happen, but then tap the back-arrow at the top. Go to your Firefox tab switcher, and you should now see a "REDIRECTOR" tab. This is the editor.
https://addons.mozilla.org/en-US/firefox/addon/redirector/
https://chromewebstore.google.com/detail/redirector/
Not only do I translate x.com -> xcancel.com, but
- cnn.com -> text.cnn.com
- youtube.com -> inv.nadeko.net
- instagram.com -> imginn.com
And more.
Regular expressions allow translations of paths for the redirection, so it does not just happen at the top level.