It's not necessarily the number of features but the process of preparing oneself for changes likely to be surprising, unpleasant, require workarounds or length

It's not necessarily the number of features but the process of preparing oneself for changes likely to be surprising, unpleasant, require workarounds or length research as to workarounds, etc. Even if it's just a matter of reading the release notes, having to do that twice as often is a non-zero burden. Of course there is Firefox ESR but I seem to remember Mozilla's website basically explicitly recommending against it with language along the lines of "This is only for people whose corporate policies require it, don't install it unless you have no other choice."