Hmm.
Mine is:
"Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:49.0) Gecko/20100101 Firefox/49.0"
I'm curious. If you go to
, is there a useragent.override set? I'm wondering if there is a string set as an override in your profile somewhere, that overrides the app's default string.
Also, I'm wondering if you're running the wrong shortcut to the browser. Like maybe the 32-bit version is "Set as Default", and when you install the 64-bit version, and after it installs, it prompts you to "Start Firefox", and it shows up as 64-bit, but then when you close it, and then click on a link, which opens the system default browser, maybe it fires up the 32-bit version.
Also, when you first install it, and it is showing as 64-bit (according to your observations), can you please open Task Manager, and click on the "Processes" tab, and see if the application is showing as "Firefox.exe" or "Firefox.exe (32-bit)"?
Then close it and re-open it, and check Task Manager again.
And please let us know the results.
If it's NOT showing "Firefox.exe (32-bit)" (in Task Manager), but IS showing "WOW64" in the UserAgent string, then that strongly suggests a prefs issue.