You should probably use Firefox ESR instead, disabling updates just isn’t an option anymore.
Naive and outright dangerous advice, every release fixes important security issues.
Then separate UI updates from security updates. No need for them to be bundled. Gradients and insane padding pushing people to not update make them inherently less secure!
It's possible to stick to an ESR release to avoid feature updates in the past, but recently, Mozilla has started bundling feature updates into bugfixes.