Hardly anyone uses it. So, I checked “anything downloaded with dooble opens up dooble”. And yes, seems to be an issue which, hypoyhetically, should be fixed by checking what dooble should do with downloaded items. You could also check your file associations in system settings as what to do with a file downloaded from dooby. There’s a good chance anything downloaded from that..euh..browser will open dooble by default.
Only for your information, Dooble is the only gui browser that do gopher.
i had something similar happen with dbeaver
after i installed it, it became the default application for opening virtually everything.
looking file associations it was listed a the top item so many file types that it would have been a royal PITA to unwind.
so i ended up uninstalling the dbeaver and that fixed everything (now using DB Browser for SQLite instead)
this is most likely something that is up to the developer of dooble to solve.
Falkon, Firefox and especially for some..opera 
Anyway, check the settings.
Go into System Settings > Applications > File Associations and search for “x-desktop” in the search field. Click the arrow by “application” in the search results list and then click on “x-desktop”.
What’s listed in the “Application preference order” table? Is Dooble first there?
Also, in Dolphin, open the settings window > “Confirmations” tab. What’s the value of “When opening executable file:”?