I know I read a thread about this some time ago, but I didn’t save it and now can’t find it. Can someone tell me how to make Thunderbird URLs open in PureBrowser?
I vaguely remembered that and it had to do with Thunderbird’s apparmor profile. See if this forum link helps your problem.
Personally, I do not like links opening automatically. I am afraid I’ll screw up. 
Otherwise, I would check for you.
I don’t use PureOS but check what is configured as your default browser, here on Arch + Plasma when I click on a link in Thunderbird it automatically opens in Firefox.
That forum article had the right information regarding apparmor configuration. I followed the directions and it fixed the problem. Thanks.