ExS, you're most welcome old chap. However:
As a result I'll never upgrade to a newer version of Firefox
I'm afraid this is untrue. Firefox LTS will automatically switch you to the next major LTS version at a certain time. Here is a handy flowchart:
If you want an alternative to Firefox, quite a good one is Waterfox, as it's 64-bit and therefore faster and more stable, and there are no forced upgrades; but it takes up a LOT of RAM! (1gb with 3 tabs open!)