"Relatedly, a number of Firefox users are actively choosing to stick with older versions because of various features being dropped, retired or (in some cases) introduced. Firefox 57, for example, is due later this year and will no longer support legacy Firefox add-ons — and many of these add-ons are a real selling point of Firefox." source https://www.omgubuntu.co.uk/2017/07/firefox-marketshare-falling-off-cliff-former-mozilla-cto-thinks-knows

Addons aren't a FF issue but a creator issue that they don't want to nor support their addon anymore to work with updated Browser that is more secure. Stick with older and get security breaches or malware infections that will happen sooner then later. And if the addons isn't uptodate then your inviting trouble.

WestEnd said

Addons aren't a FF issue but a creator issue that they don't want to nor support their addon anymore to work with updated Browser that is more secure. Stick with older and get security breaches or malware infections that will happen sooner then later. And if the addons isn't uptodate then your inviting trouble.

I understand security first, but if the addon I wanted to use is not working in FF (or unavailable because the addon developer stops it), I would already not be using firefox and even if FF is the best security browser.

It is FF Quantum creator making it hard for addon developers to rewrite the addon, and many addons who stopped supporting quantum are those very popular addons.

"Quantum requires old extensions to be essentially completely rewritten. If their UI was using XUL it has to be redone in HTML. The Javascript APIs that old extensions used were all deprecated, and the WebExtension APIs only provide a (growing, thankfully) subset of the functionality." source https://github.com/downthemall/downthemall/issues/270

Anyway I happen to see a solution and that is to use Waterfox. Hope Waterfox works forever. "If you have 64bit ... really there is no point for me to use this OTHER than THE SUPPORT for addons pre-Firefox 57 .... but ... Waterfox is a good option .... IIRC it doesn't have alot of that extra crud (I say that loosely/half joking) that Mozilla has been adding on to Firefox too ... I looked at it after a few years of not using it and i was "oh ...that and that ... and this other thing too ... that's handy" ... it's here on github too https://github.com/MrAlex94/Waterfox ... I can't 100% speak for how to date with Mozilla security fixes it is ... not saying anything one way or the other until i have more info" "You can install Waterfox. It will keep supporting full extensions." source https://github.com/downthemall/downthemall/issues/270

I accidentally choose the answer by WestEnd as the choosen solution and unable to undo it. If anyone have any other answers do post it. Thanks. Update: I saw the undo option, ignore this post.