I think your distribution includes platform-specific extensions, so I suggest testing with those disabled in case there is a compatibility issue with Firefox 38. One way to do that is to run in Firefox's Safe Mode. That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

If that doesn't make any difference, you could try adjusting the setting for High DPI support, which operates as a global zoom factor:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste pix and pause while the list is filtered

(3) The layout.css.devPixelsPerPx preference usually is set to -1.0 which means use the system default. You can change that to 1.0 for 96-dpi dispay and see whether that is what you prefer. And you can increase the size from there in increments, for example, 1.1 for a global "110%" zoom.

Please be careful not to set this value to something tiny, as it can be hard to recover from that.

Another Ubuntu user posted about this last hour: Firefox graphics scaled up ridiculously after update (unsolved).

For what it's worth, setting layout.css.devPixelsPerPx to 1.0 fixed it for me.

Same here.. Safe mode did nothing, but layout.css.devPixelsPerPx was set to -1 .. changing it to 1 fixed the problem....

Thanks all!!!

Win 7 64 same issue, though not as bad as OP, recent update to38 default zoom appeared 110%, so was zooming out to 90% for 'normal' appearance. Tried Safe mode showed no change- changing the about:config setting from -1 to 1 fixed.

I had this problem with FF38 on Ubuntu 12.04 and after hours of searching I finally found this thread! Changing layout.css.devPixelsPerPx to 1.4 returned to the normal appearance before the FF38 update. Thanks!

@jscher2000 what do you mean "be careful not to set this value to something tiny, as it can be hard to recover from that." ?

If you would set this pref to 0.3 or even smaller then the entire Firefox user interface would shirk to a very small window. You would have to edit the prefs.js file and remove or modify the corresponding line to recover from this.