| Installing old addons in firefox 51.0 so, my harddisk was on it's way out, was making a clicking noise but functioned well. so I decided to try to ghost it to a new harddisk, that failed because a readwrite head musta been busted. sigh. so I got a new copy of windows 7 installed and then needed to reinstall all my software. Firefox 51.0 is the last firefox that you can stop from updating that allows flashplayer. flashplayer was fairly easy to install. albeit I have unbricked 32 instead of 31 set to version 33 like before (to disable the "outdated plugin" message) but all my other precious addons were no longer available! luckily I had a backup computer with all the .xpi files and a working firefox 51.0 installation so I could copy some files over, and if you get an xpi you too can install it in an old firefox by the following: in firefox, go to about:config and search for "xpi" then toggle xpinstall.signatures.required to false. you might need to create a folder called "extensions" in "C:/Users/yourusername/AppData/Roaming/Mozilla/Fi refox/Profiles/yourprofile.default/" in this extensions folder insert your .xpi files. now in C:/Users/yourusername/AppData/Roaming/Mozilla/Fir efox/Profiles/yourprofile.default/ you need to modify the addons.json and addonStartup.json.lz4 what you don't have addonStartup.json.lz4? you'll need to copy it from an older ff51 install that includes the addons. if you don't have that it is a binary file so I can't help. you could try without it and see if it auto generates this file on browser restart after accepting addons you installed this way. when you open firefox after this, it will open tabs asking if you want to accept the addon, check the box and accept. congrats your old addon works. some addons like adblockerultimate may force you to update that one addon before the browser stops disabling it. but things like downthemall the download manager, or advanced cookie manager and older versions of noscript install this way just fine. | 
