I take it the new profile is working without a problem, yes? ''When you are ready, we can start moving parts of your'' ''old data and '''Copy''' it to the new profile.'' ------------------ First; open your file browser to the '''New''' profile; Type '''about:support'''<enter> in the address bar. Under the page logo on the left side, you will see '''Application Basics. ''' Under this find '''Profile Folder. ''' To its right press the button '''Show Folder. ''' This will open your file browser to the current Firefox profile. Now '''Close Firefox. ''' Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder '''Linux: ''' Under the page logo on the left side, you will see '''Application Basics.''' Under this find '''Profile Directory. ''' To its right press the button '''Open Directory. ''' -------------------- Second, open another file browser to the '''Old''' profile. You should see it on the directory "tree" in the first file browser. Right-click on that folder and select '''Open In New Window. ''' -------------- '''Warning ! ! ''' Only '''Copy''' the files you are told. We will avoid anything that might cause problems. Here is the list of files; '''cookies: ''' cookies.sqlite Do not bother. Cookies will be created when you return to the website. However, you will have to log in everywhere. And you may lose any settings that websites store on your computer. '''extensions(add-ons, themes): ''' it would be better to download them fresh. If you need to remember what add-ons you had, open the '''extensions''' sub-folder and look at the '''.xpi''' files ++++++++++++++++++++++++++++++++ '''logins.json''' and '''key3.db''' [v58+]'''key4.db''' : These are the password files. '''Copy Both ! ''' '''persdict.dat''' : custom dictionary - words you added to the dictionary '''mimeTypes.rdf''' [v55+]'''handlers.json''' : What to do with what type of file. '''places.sqlite''' : History and Bookmarks. '''prefs.js''' : Your settings. '''Note:''' If you were having problems, this could be contaminated. '''cert8.db''' [v58+]'''cert9.db''' and possibly '''cert_override.txt''' : Stores all your security certificate settings and any SSL certificates you have imported into Firefox. If removed, the '''cert8.db''', '''cert9.db''' file will be replaced. '''search.json.mozlz4''' : file stores the user-installed search engines that are available in the Firefox v47+ '''search.sqlite file''' and '''searchplugins folder''' store the search engines that are available in the Firefox Search bar '''before''' Firefox v47 +++++++++++++++++++++++++++++++++++++++++++++++++ '''Copy''' these files from the old to the new. You may get a message that the files already exist. Choose to '''over-write''' for each file.

I found cert9.db and places.sqlite but I don't know the location that you seem to mean in "old to new". Which folder is the "new"?

'New' refers to the new profile.

Thanks. I used the info you provided to eventually find the files here: C:\Users\<windows-user-name>\AppData\Roaming\Mozilla\Firefox\Profiles