Thanks for reporting!
This is quite likely solved by having PureBrowser provide firefox-esr.
Show. The installation Worked =) thanks.
but... a new problem appeared. See...
Please (if still an issue now several years later) report that other issue as an independent issue report. Thanks.
Also on a related note, Purism has abandoned the development of PureBrowser for some time now.
PureOS now recommends using GNOME Web (a.k.a. Epiphany), and also provides Firefox ESR.