Please contact the website owners to inform them of this problem.

I'm kind of a newbie, and have never seen an .xul file in my life. My grandmother has an .xul file that she's trying to open, but it won't work. Stupidly, I updated the OS from Snow Leopard (10.6) to El Capitan (10.11) and I'm not sure if that was the source of the problem. I looked everywhere for a solution, or some way to open it, but all evidence pointed to Mozilla Firefox. When I double-click the .xul file, it opens in Firefox and the error message reads: Remote XUL This page uses an unsupported technology that is no longer available by default in Firefox. Please contact the website owners to inform them of this problem. Okay..? I tried installing the Remote XUL File Manager to help me with this. I tried to add the domain by copy and pasting the url of the file, which is "file:///Volumes/ANCESTRY/HannahLane1940census%20-%20Copy%20-%20Copy.xul". But I get: XML Parsing Error: undefined entity Location: file:///Volumes/ANCESTRY/HannahLane1940census%20-%20Copy%20-%20Copy.xul Line Number 586, Column 7: <checkbox id="trackingProtection" ------^ When I open it on her Windows laptop, the file extension changes to .htm, which I assume is because Macs don't support .htm or .html file extensions anymore. When I try to open that in any browser, I get: %brandDTD; %globalPreferencesDTD; %preferencesDTD; %privacyDTD; %tabsDTD; %searchDTD; %syncBrandDTD; %syncDTD; %securityDTD; %sanitizeDTD; %mainDTD; %aboutHomeDTD; %contentDTD; %applicationsDTD; %advancedDTD; ]> I have tried installing previous versions of Firefox, (I have 44) but to no avail. I have also tried running it in safe mode, and completely restarting and refreshing the Firefox application itself. Can anyone help me? I honestly feel guilty for updating to El Capitan, and I hope there's a way to use Remote XUL Manager so I can open this file.