[1] "Choose default apps by file type"
[2] "Choose default apps by protocol"
[3] "Set defaults [both file type and protocol] by app"

Some variant of this question gets asked a lot - here and in other forums. (Actually, the problem is more with Windows 10 than with Firefox.) The behavior goes like this: User installs Firefox, ticks "Always check if Firefox is your default browser" and in general does everything to try to make it default web browser (Settings > Apps > Default apps > Web browser) - except that <i><b>Firefox is not presented as a browser option</b></i>. In this situation, the "standard" solution < https://support.mozilla.org/en-US/kb/how-change-your-default-browser-windows-10 > fails on step 5, for the reason just given: the "Choose an app" box does not list Firefox as an option. From that point, most of the threads look for a workaround, but diverge based on <i>different versions of Windows 10</i>. Unfortunately, the next two paragraphs are needed to explain why that is the case. Windows 10 is a bit of a mess [it's still Windows, after all], and an ongoing aspect of its development is replacement of the Control Panel with the Settings app (via the Win-X menu, which also points to the Settings tree Home page, as well as other Settings pages and other management apps currently outside the Settings app). So there's a mix: sometimes a function can be done in either place, sometimes Settings are executed via legacy CPL applets, and sometimes CPL elements point to the corresponding (hopefully equivalent) Settings element. As development continues on the Settings app, and CPL applets are removed, sometimes aspects of a CPL applet's functionality are simply not replaced, or not replaced in equivalent ways. IOW, as this process continues, releases will differ in how things can be done. Unfortunately people are not generally aware of this, particularly as 4-digit version numbers and 5.3-digit build numbers (= 7-digit KB numbers) have replaced older and simpler version numbering schemes. This problem is compounded by automatic updates and, in the update history listings, the build number is replaced by the KB number describing (often inadequately) the changes in that build. It's a terrible mess for support of 3rd-party applications like Firefox -- and unfortunately this information is not even collected by Mozilla's troubleshooting data collection. (That is why the title is so specific about version info.) For example, question #1167367 (2017-0711) ( https://support.mozilla.org/en-US/questions/1167367 ) found that he could get around the lack of Firefox option in the "Settings > Apps > Default apps" by going to the corresponding "Set Default Programs" CPL applet, which allowed any program (now "app") to be set as the default for all file types <i>and</i> protocols it can handle. But that is no longer possible with the current version, for which that CPL applet is removed. Meanwhile, in the current W10 version, "Settings > Apps > Default apps" currently handles that CPL applet's functions via three sub-heads at the bottom of the page: [1] "Choose default apps by file type" [2] "Choose default apps by protocol" [3] "Set defaults [both file type and protocol] by app" Now, even though Firefox is NOT listed as a choice in the "Web browser" "Choose an app" box, it IS listed as a choice on those subhead pages. Specifically, for: [1] Firefox is listed for most of the desired file types. (Where relevant file types do not offer Fx as an option, that probably reflects a fault in the installation script.) [2] Firefox is listed for the desired protocols -- and when you select Fx for "HTTP", it is then listed as the default "Web browser". So <b>this solves the problem</b> -- even though <i>Fx <b>still</b> is not listed as an option in Web browser's "Choose an app" box!</i> [3] Firefox is listed here - in fact, it might be listed multiple times! In my case, it is listed 4 times - no doubt reflecting the times I uninstalled and reinstalled as I tried to get it to show up in the Web browser "Choose an app" box (which never happened). <b>So this is a workaround, not a proper fix. It reflects an obviously buggy registry and system, but does invoke Firefox when needed.</b> Similarly, in #1172782 (2017-0824) ( https://support.mozilla.org/en-US/questions/1172782 ), the "chosen" workaround was "Start > Control Panel > Programs > Default Programs > Set your default programs", but in my version that just takes you back to the same point (Settings > Apps > Default apps) and so is of no help. (Arriving by a more circuitous route does no magic: Firefox is of course still not an option.) User "Donaldas" made that point, but was advised to start a new thread (which unfortunately did not happen). So that remained unsolved in that thread -- but the solution (at least for the current version) is described above. More recently, in #1186804 (2017-1117) ( https://support.mozilla.org/en-US/questions/1186804 ), Catherine Barber posted the problem and at least recognized the need to identify her W10 version (Home v:1703). As with the other questions, people who were trying to help ignored her basic point: that Firefox was NOT listed as an option for Web browser "Choose an app", and so her final comment (Feb.28) was "If Firefox isn't listed, how can I set it to be my default browser - or, isn't this 'allowed' in Windows 10 ?" Again, though, a solution for W10/Pro.v.1709 (10.0.16299.248) x64 is above. Unfortunately, with the bugs in W10 (illustrated by failures to register on install or de-register on uninstall), combined with the constant churn in the Control Panel to Settings migration (without first fixing the bugs), it's impossible to guarantee that solution will be OK for any other version. As I said, Windows 10 is a nightmare for 3rd-party software support. To minimize that nightmare, we at least need to be aware of, and sensitive to, the complications. That's why I wrote this detailed (<b>TL:DR</b>) explanation. ''fixed the hyperlink in the 2nd paragraph and another added in the 5th paragraph - fixed by a moderator''