Firefox's privacy.resistFingerprinting option reports a very old User-Agent (50.0)
If your copy of Firefox has been behaving oddly but you can't figure out why, you might want to check the value of your
Starting around Firefox 41, a setting called
privacy.resistFingerprinting was introduced. This option didn't get much fanfare until the release of Firefox 55, when Mozilla announced a litany of features they'd be retrofitting into mainline Firefox from the Tor browser bundle. I decided to enable
privacy.resistFingerprinting in Firefox 56, hoping this preference might obviate the need for one or more of the privacy extensions I use.
After a few days I forgot all about toggling that option. Meanwhile, Firefox had begun acting strangely:
Some sites using Flash were telling me I didn't have Flash installed, despite being in my Flashblock whitelist.
Some sites had miscellaneous elements that stopped functioning properly.
When I started or restarted Firefox, it came up windowed instead of maximized, even after verifying that
xulstore.jsonhad the main window set for
- I went to install a Firefox add-on directly from AMO and got the error message: "This add-on requires a newer version of Firefox (at least version 56.0a1). You are using Firefox 50.0."
At this last point I knew something was seriously amiss, but I still hadn't put it all together. Some searching turned up this Reddit thread with the answer. Disabling
privacy.resistFingerprinting not only fixed the User-Agent string but also resolved the other weird symptoms I was seeing. It turns out this setting touches a lot of code, and changes or interferes with things I didn't expect it to.
Unfortunately, you can't enable both
general.useragent.override at the same time. Firefox explicitly disregards a custom User-Agent setting when the
resistFingerprinting toggle is true. Forcefully regressing the User-Agent from 56.0 all the way back to 50.0 doesn't work for me, so I had to disable the new option.