The code used in complete themes can no longer be applied by an extension in Firefox 57+. However, some of their style rules and images could be applied using custom style rules. Is there a particular one that's important to you?

jscher2000: I have six themes that were usable before the new version of FireFox that I used all the time. I rotated them one per month. I also lost the ability to use Foxit PDF Creator, Logitech SetPoint for the Mouse and Keyboard, and Iobit Surfing Protection. It's starting out like Windows10.

chienetfou said

I have six themes that were usable before the new version of FireFox that I used all the time. I rotated them one per month.

That's a lot to convert.

I also lost the ability to use Foxit PDF Creator, Logitech SetPoint for the Mouse and Keyboard, and Iobit Surfing Protection. It's starting out like Windows10.

I don't know whether Foxit will create a new add-on, but there are many other PDF add-ons you could look at. A couple months ago I made a starter list here:

https://discourse.mozilla.org/t/so-many-pdf-converters/20193

What did SetPoint do?

What did Iobit Surfing Protection do?

jscher2000: SetPoint is the Logitech control GUI for the settings for the mouse, and keyboard if it's wireless also. Iobit is a Surf protection that stop and/or alerts you to phony/malicious sites, phishing, redirects that might be suspect. It works well in conjunction with AdBlock Plus. Also works well as a stand alone.

What is involved in convert/applying the old rules and images? I'm not a coder but I have been know to supplement my pay with general IT work.