I should add how I know it's a programming error: After I RESET firefox - and having to re-do all customizations of course :-((( - the first load was clear and nice readable texts as usual, but within less than a second firefox CHANGED all texts the way I wrote: greyish, smaller, and worst, somewhat blurred. In short: pretty much UNreadable.

Thus: There is some command in the update that creates this blur after launching firefox. Hope this helps?

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

You can check for corrupted and duplicate fonts and other font issues: http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Separate Issue; Your System Details shows;

Installed Plug-ins

Shockwave Flash 18.0 r0 Shockwave Flash 16.0 r0
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Note: Edge on Win10 does not use ActiveX version but its own Flash.

Flash: Grab the uninstaller from here: Uninstall Flash Player | Windows Uninstall Flash Player | Mac

Then reinstall the latest version.

Flash Player Version 18.0.0.232
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users must download the “Internet Explorer” and “Plugin-based browsers” installers.

You can try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Try to set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. If necessary adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right.

You can look at the Default FullZoom Level or NoSquint extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

Check that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

Is there a reason why you are Testing the Beta channel builds where there are some things still being tweaked and not the stable Release which is currently Fx 40.0.3 at www.mozilla.org/firefox/all

The Beta builds can get six to twelve builds for a version on Beta channel.

Thanks for your reply James, that's a good point. I never thought about that but I guess I am getting "beta" releases because I must have clicked once upon a time "Become part of our beta release testers" or so. You are completely right I should quit that and only get the standard releases to avoid more trouble.

Let's leave this ticket open though, so that your beta colleagues become aware why they shouldn't roll out this build.

-- Edit: Oh, only now I see that two other members have replied as well, thank you (I didn't get an email for those earlier, only for James'). Sadly, it is none of your (otherwise good!) points. Fonts are fine, hw acceleration, etc nothing has to do with it. James hit the point: I am using a beta version, and obviously they have bugs. I will change to normal versions. Thanks so much for your kind replies!