i do not use vivaldi as default browser. i would like it to be the default browser for hyperlinks in rss feeds and email from the vivaldi forum. i would like firefox to be default browser for any other program. is this possible in linux/windows?
First of all, some programs will allow you to select file associations at time of install, but no program can specify what the source of the call is, as you ask. In general, however, file associations are set up in the OS, not within individual programs.
I don't know about Linux (don't use it enough) but Windows settings cannot be made that granular. You can (in Windows) select Vivaldi to be your default application to open hyperlinks, but you cannot restrict this to "only rss feed hyperlinks." Windows cannot distinguish FROM WHERE the command is issued, in setting up defaults. It can only distinguish WHAT IS BEING OPENED. In other words, Windows doesn't know and doesn't care who asked to open a web page, follow a hyperlink or open an email file. It only knows/cares what app is supposed to respond when a certain type of file is to be opened, or a certain protocol executed.
So, no, I don't think what you ask can be accomplished. Not by any program on any system.