Sometimes it works, sometimes it does not for the same URL. Not stable at all. Plugin seems to work ok for getting the results, it is the site that sends the results that has the problems and needs to be updated.
I've been looking for a replacement for CZ.NIX Labs' DNSSEC/TLSA Validator, which I still use in Cyberfox.
For Firefox Quantum, I found Popineau's DNSSEC unsatisfactory at least until the promised DNS over HTTPS (DoH) service is fulfilled at which point I'll re-evaluate. But it's been three months since their last update...
So, upon an icon click, HTTPS+ Checker queries a server (port 443) in France and presents a color coded result to the four fields.
As such, and given its experimental nature, the ability, validity and security of this extension must rest upon the user's confidence in AMZG_Beu's expertise, methodology and strategy. And, of course, that of Mozilla's vetting.
At this point I can't find reason to doubt that confidence and give five stars for effort, implementation and apparent efficacy.
I look forward to continued development and success. Good luck and thank you.