HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.pdf 

         HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.ogg

Do I have to edit this stuff out by hand?   If that is the case, could someone provide me a list of the places where Firefox has fiddled with the existing class keys?

I have reviewed previous questions about this problem, but they do not address what was done, and how best to repair the hijacking of the filetypes. There are also Bugzilla bugs that suggest this issue was addressed, yet with Firefox 46, my .PDF files have been redefined as "Firefox HTML Documents", once FIrefox was set as my default browser. I'm running a Windows-XP/SP3 box, with latest Firefox 46. All my .PDFs were converted to display as "Firefox HTML document" in Windows folders. This is a documented bug with Firefox 31, from 2014: https://bugzilla.mozilla.org/show_bug.cgi?id=1069375 and https://bugzilla.mozilla.org/show_bug.cgi?id=1049521 What seems to be happening is that Firefox 46 (or maybe ver. 43? or ver. 45?, as I started with one of those, I think..) is setting values in Windows Registry, for .PDF filetypes, as well as other file-types, such as OggTheora files (audio files with .OGG extension) to "Firefox HTML". These string values are getting slotted into the registry as part of making Firefox the default webbrowser, which is probably something you don't want to do, since that seems to make FIrefox needlessly re-define file associations for your filesystem! I already have a .PDF reader - Sumatra - which works well for my purposes, and I have to stress that Firefox should *not* be fiddling the file association values of the Registry as part of it's install. This is not necessary, the thread info in both the bugs indicate that this was viewed as intrusive and incorrect behaviour, which was indicated as having been corrected. (Patches developed, bugs flagged as "Verified Fixed") I can use control panel/folders to reset the .PDF and .OGG file associations, but what do I need to do to remove these inappropriate "Firefox HTML" associations that now pollute my registry hive? Using Registry editor, I have confirmed that even after I set the .PDF filetypes to <blank>, the registry key "FirefoxHTML" remains assigned to registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.pdf as a subkey of "OpenWithProgids". Same for key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.ogg Do I have to edit this stuff out by hand? If that is the case, could someone provide me a list of the places where Firefox has fiddled with the existing class keys? This action seems to be associated with making Firefox the default webbrowser. I have also tried some of the suggestions others have made, but these to not correct the inappropriate registry associations. If folks are unable to address this problem by code correction, could I at least get a simple list of the registry keys that have been fiddled? I can delete the "FirefoxHTML" subkey that has been defined for HKLM\SOFTWARE\Classes\.pdf, but I don't want to alter the registry hive without fully understanding where all the inappropriate registry changes have been made. Installation of a Webbrowser should not mess up registry definitions for document files. I don't want to use Firefox as a .PDF reader, anymore than I want to use Adobe Acrobat as a webbrowser, or an iPhone as a secure communicator. Is there an about:config option which can be altered/set to remove the inappropriate registry-based filesystem redefinitions that Firefox has made? Thanx for any info. If I don't or can't get an answer on this, and if I have time, I will dig deeper into the problem, and see if I can determine a resolution that is a bit more sophisticated than just blanking or resetting the .PDF definition with control panel/folders.