i've noticed some interesting things when testing on dif plaforms and with various dif file extensions.. different browsers use some different strange tricks for different files.
two things: don't be surprised if after you set a browser to be default, the browser itself still reports that its not the default browser, even when it behaves as if it is. you can try letting it set itsefl to default once, but if it always pops this up when you change browsers back and forth, then just disable the "check if default" for the browser, since its never going to be satisfied; browsertrayswitch doesnt try to change every possible registry setting that a browser might look for in order to satisfy itself.
ive noticed on a new virtual machine that mozilla was complaining about double clicking on .url files, untill i let it make itself the default browser once; after that i could use browsertrayswitch to switch between browsers and double click .urls without issue.
so when in doubt, let each browser make itself the default once - this may let them set up the uniquely wierd settings they want. BrowserTraySwitch should thereafter be able to switch between them without issue.