The problem is not with the theme I use, but how the site organizes them and the lack of incentive for those themes to continue to be developed. Honestly, I like to install full themes and always wonder if this does not detract from performance in the browser and give a nice look, such as Walnut for example that is my favorite. The site does not hide the complete themes that are incompatible with the current version of Firefox and I think that the developers of the site should pay more attention to this detail.
Since Firefox started doing fast releases, developing and maintaining themes has become very difficult. There is currently a plan to replace them with a new theme API, so there are no plans to fix any complete theme bugs on AMO.
Speaking that one day I saw a rumor that it will no longer be allowed to use full themes in Firefox left me worried simply because I like this kind of theme. The only thing that Mozilla has missed is to adopt the same refresh cycle and the look of Google Chrome (note that both browsers look alike), which has accelerated the development of Firefox and Mozilla has little time to make a complete browser available New features and quickly, leaving the addons and complete themes incompatible because some developers give up to continue their work because of this development cycle.