It might be fast, but the developers seem to have forgotten something.
I used to be able to scale the viewport by dragging/pushing one of the edges. That seems to be a thing of the past. Now you have full width and a smaller width, which is defined to be the width you last used before the upgrade, and on my laptop that happens to be something like 300px or so.
Has anybody found a way to change this? Without going back to the previous version? I'd be very interested to know. Ffx has been my best buddy for at least 10 years, and I have add-ons that I now can't use.

I can drag the edge on mine. FX Quantum ver 57 Which addons don't work? Some times the addons take a while to get updated.

I have the same version (64 bit). I'm not able to widen or narrow the viewport manually by dragging. My most important add-on is Web-Developer. I can use it when the widow in covering the screen, but not when it is at 300px. And I can't get those 300px changed!

I find I can still use Ctrl-Shift-m with it to see the effect at various screen widths: it even shows the exact number of pixels in the width.

Frank

If I have the browser window maximized, I can alternate between the full viewport and my 320px window by using that key combination.. If I 'middlemize' the wiewport, to those 320px, I only see that narrow view when using the same keys. See this image: http://eikweb.com/stuff/Ffx.jpg to see how much of that web developer toolbar I can see.

Maybe it's me. Everything seems to go against me these days... (That was off topic).

I would reinstall.

Yes, I will when I get home. I have just a tablet with me now.
But it is strange that this should happen on both desktop and laptop.