Hi, do you mean (A) your system-level enlargement is 125% and Firefox is implementing that, or (B) your system-level enlargement is 100% and your default page zoom is customized to 125%?

For System-Level Enlargement

If you try manually overriding the enlargement level, is there any difference? Here's how to test:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste layout.css.devPixelsPerPx and pause while the list is filtered

(3) Double-click the preference to display an editing field, and change the value to 1.5 (without the minus sign) then press Enter or click the blue check mark button to save the change.

Be careful not to omit the period or everything will be unusably large.

What you should expect: on a regular display, the font size should be 150% of the standard 96ppi resolution. On a double-density (Retina) display, it would only be 75%.

Maybe a more realistic value would be 1.2 (120%) or 1.33 (133%). I'm suggesting these rather than 1.25 because they are the standard zoom steps in Firefox.

Any difference in rendering quality?

Note: The default value of -1.0 allows Firefox to use your system's default enlargement percentage.

No, this does not help. Something got broken with the recent Firefox update. That used to work for years just fine. This new setting simply enlarges the fonts, but they are still pixelated and garbled.

The value for this parameter was indeed -1, as you said.

Its (B) in your question: The resolution is native 2560x1440 (Quad FD, sometimes also called 2K display) and the scale is set to 125%. I have it so for several years. This Firefox issue is brand new.

I would be happy to revert the Firefox update, if I could.

It's a little complicated downgrading, but check out: Install an older version of Firefox. Normally, you need to create a new profile, so having your data in Sync well in advance is recommended.

You might test whether this is a graphics card incompatibility. See the following article for steps to disable hardware acceleration: Firefox's performance settings.

I installed the ESR version, and that does not have the bug. Web pages are ok again. Of course, the card is ok, the only issue at hand here is to ask Firefox release to run the diff to the previous version, and make a corrected release.

There is a method to diff down to the breaking patch, which is to run Mozregression. If you have the time and patience to run it on your system, that would be great, since most people do not have QHD monitors. But it's possible someone else already did that and filed the bug. In case you can pursue further:

• Bugzilla (for searching/filing): https://bugzilla.mozilla.org/
• Mozregression tool: https://mozilla.github.io/mozregression/