I personally don't see a good reason to opt out of security changes for any longer than strictly necessary and that's exactly what you do when using ESR. First they get backported by Mozilla. That's after normal users receive them. This takes time. Then testing takes place because they don't want to push out a hasty fix for ESR. This takes more time. And then third party packagers need to pick up the changes and repackage (which is mostly pointless and adds more time). And then eventually it rolls out days/weeks/months after normal users have long received the patches. I don't seen any good reason for such lengthy delays for normal users. In some big companies where they manually review updates for workstations it's a compromise between the extra work and stability. But the tradeoff is timely access to security fixes; which ESR simply doesn't provide.
Or missing out on new anti-fingerprinting and anti-tracking improvements. Note that adblockers don't generally do the former.
Note, the "normal users" you describe are involuntary testers who got forced into the role because keeping testers on the payroll is so last century.
Insisting on an ESR release on what is effectively a poorly supported type of operating system where every install is basically a snow flake that heavily relies on it's users being able to fix all sorts of weird issues (i.e. every Desktop Linux distribution ever) is a bit weird and overly selective.
As for Debian's standard response to using non-Debian packages, they have the following wiki entry:
Most companies don't prioritize legacy support, so there is no point in developing for Firefox ESR.
On the other hand, the more legacy you support, the larger your potential customer base is. Also, if everyone else in the sector only supports the most recent release of only two browsers, you might have that extended customer base all to yourself.
> you would better develop your app using the latest dev tools and targeting the latest web specifications only.
That is one school of thought.
Another is that you should regularly use the oldest platform (hardware, OS, browser) that you want to support. That way you won't get to two weeks before release, decide you ought to test on a dual-core 4Gb machine like you were planning on supporting, and realise that although it technically runs, it's so frustratingly slow that no-one would want to, but there's not enough time to make the changes you'd need to get it working "acceptably". OTOH, if you were using it on hardware where it ran like a dog for you from the 3rd sprint, you'd probably have got round to making the changes for it to be acceptable on that era platform.
It is more a decision about the trade-off between costs and benefits. Supporting really old browsers like IE is a pain in the ass.
> Another is that you should regularly use the oldest platform (hardware, OS, browser) that you want to support.
It is possible to manually slowing down your browser with dev tools. https://imgur.com/a/99XbwN9
Or perhaps even made saner choices in the first place rather than rambling about "premature optimization".