After months of training and you finally understand all of a program’s commands, a revised version of the program arrives with an all-new command structure.
Thoreau’s First Theory of Adaptation
As usual this comes without warranty, but given that Firefox latest update did prevent some of my favorite Add Ons working, I switched to the ESR version (Extended Support Release). In contrast to the regular version of Firefox, the ESR version has fewer updates (save for important security updates). And as written, add ons like DownThemAll! or Video DownloadHelper still work without a hitch (and without being marked as legacy).
True, you cannot prevent the inevitable. Once there is a new release and the old version is no longer supported (security updates!), it becomes time to switch. But I hope when that time comes (around March to June 2018) the Add Ons will also have gotten updated.
So, while not meant for “normal” users, the ESR version is worth a look if you are frustrated by constant updates that break your toys tools.