It definitely does not work indefinitely. If I recall correctly, several years ago a failsafe was added where very obsolete versions were assumed to have update check failure and were forcibly updated. Or something like that.
If you want a version that has fewer major updates and bug fixes, you could look at the Extended Support Release. The ESR based on Firefox 91 is nearing its "end of life" and will be moved to an ESR based on Firefox 102 in the next couple of months: