(In reply to sedan-01-unset from comment #3)
By this you mean that letterboxing is turned on? It is turned on for me.
Additionally, no letterboxing will be applied if the window is smaller than the lowest letterboxing size. So try a smaller value like 400x400 and ensure your window is larger than that.
Not sure I follow. The default size seems to be 1000x1000. Which is unchanging no matter what the dimensions are set to privacy.resistFingerprinting.letterboxing.dimensions. Id like the window bigger not smaller.
So there are two window-size preferences. privacy.resistFingerprinting will make the outer size of new windows (i.e. popup windows) come up with specific dimensions. Letterboxing (privacy.resistFingerprinting.letterboxing) will make the content size of windows grow and shrink in 'steps' as you resize your window. When no value in privacy.resistFingerprinting.letterboxing.dimensions (or an invalid value) letterboxing will step in 50, 100, or 200 pixel ranges depending on the size of the window.
If you don't see the content window being letterboxing (the gray outline around the content in this image when the dimensions pref is empty; then letterboxing is not enabled or working for you.
The change will only take affect after you refresh a webpage (like mozilla.org). I just tested with 800x800 and it worked for me; but 8000x8000 make it appear as it letterboxing was disabled.
When you say appeared as if it was disabled, do you mean it appears as 1000x1000 or it appears full screen? I don't think refreshing a page changes the size of your window - restarting firefox should see the change take affect.
I mean it will appear as if there is no letterboxing - the content will fit the size of the window with no border.
The dimensions pref is intended to hold a list of values (and as I mentioned is really just for testing so it's real annoying to work with). Setting it to e.g. 400x400 (and making sure your window is larger than that) should show your content with giant borders around it.