If you haven't already... To see whether one of your extensions is involved, could you test 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.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Firefox has larger tab image graphics for "high DPI" displays. But I would expect the size to be determined once and not change when you hover or stop hovering... but to pursue that thought:

• What is your default zoom level in Windows? For example, it may be 100%, 125%, 150% or a custom value.

• If you have manually customized the Firefox preference "layout.css.devPixelsPerPx", what is its value?

To check that second one:

(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 search box above the list, type or paste pix and pause while the list is filtered

(3) Find layout.css.devPixelsPerPx on the list.

• A value of -1 means Firefox will use your Windows zoom level to size the text and images on your toolbars.
• Any other value is applied instead of your Windows zoom level, multiplied by 100, so 1.2 gives you text and images 120% larger than the old standard.

Please don't modify this value now, that will only make things more confusing.

It does not exhibit the behavior when restarted in safe mode. It appears that Tab Mix Plus 0.4.1.3.1 is causing the problem.

Thank you for your help!

Alan

There's no official update for TMP, but there is a much more up-to-date test version available on the "Versions" page: https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/versions/

I haven't tried it myself, but if you miss some features of TMP, maybe give it a shot.