Hi,
I have always used themes in my browser and never had any probs with doing so. I wanted to use one tonight (realizing mine wasn’t working anymore… for some time now) and when I checked them out I found there are VERY FEW for my version of Firefox! Anyone know WHY that is? I update my version when asked to do so and can’t imagine why MY version doesn’t allow for a large assortment of themes. I’m running version 41.0.2 Thanks in advance for any suggestions. 
Are you talking about Complete Themes or Lightweight Themes? Complete Themes are decreasing in number because it can be very difficult to keep up with the Firefox release process, and they require changes for pretty much every release. Lightweight Themes should work fine in all versions of Firefox, though.
That’s why I am here. Just got updated to 42.0 and my Lavafox theme that gives me a dark page upon opening a tab, page, etc. has stopped working and I have not found any so far that work. A bright white window on a large monitor is blinding.
As mentioned it’s hard to keep up with the updates is understandable. But I am finding the updates are causing more problems. I’m about ready to revert or move on to another browser.