In a couple of questions similar to this (#1172782, 2018-01-31 and #1186804, 2017-11-18), posters have noted that the stock solution for making Firefox W10's default browser does not work, because FF is never offered as a choice in the Windows 10 default-app preferences. This is a particular problem for users not yet ready to move to FF Quantum (having not yet found replacements for useful extensions), because the only other option offered by W10 is to search the W10 app store, for which Quantum is the only FF listed. Unfortunately, in both of those cases the question has been closed as solved, even though this problem of the missing default-app option for FF was not solved. I ran into this problem. After updating from FF56 to FF58 and losing too much functionality (including TabMixPlus session stores), I tried to move back to FF52ESR (to try to fend off problems until I can get some work done) and ran into new problems: loss of all bookmarks (though they are still in the folder - and I will find out how to recover them inside FF) and loss of FF as default browser, because it is not offered as an option in the W10 default-app settings. Presumably this is (yet again) a Windows Registry issue -- somehow the installation of FF52ESR fails to set something in the registry -- so that FF is not presented as one of the choices for default Browser app. I want to highlight this issue, with more definition than I have found it given elsewhere, in case anyone has insight into this. Meanwhile, if I come up with the solution, I will also post that here.