I have a client certificate from Cloudflare, on Android and Linux it always works (with Chrome). But I mostly use Linux with Firefox, and there it works sometimes and sometimes not 
I have already deleted all data (cookies etc.), and used the private browser, has brought nothing.
The weird thing is that it works after a while. e.g. I start firefox in the morning and want to go to the corresponding page and it doesn’t work, but when I do something else and try it again later it works, and when I do something else again (in firefox or other programs, it doesn’t matter) it doesn’t work again.
And it doesn’t matter if I use the private browser or not.
I have also already replaced the certificate and I have tried a legacy pkcs12 certificate, all has done nothing
(All certificates were in pkcs12 format)
Does anyone have any idea what this can be?