Hi, is rather strange. Am not sure of a good answer but let's start here : https://www.malwarebytes.com/mac/

Hi mart345, unfortunately the screenshot didn't upload successfully. Can you attach the image file to a reply? https://support.mozilla.org/questions/1232745#question-reply

Yes, here it is.

Oh wow, that's an interesting side effect!

One possibility is that it's a theme add-on. You could check for that on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Themes

The themes included with Firefox are Default, Light, and Dark. Anything else is an add-on.

Was that it?

It's also possible that you have an unexpected extension.

In the left column of the Add-ons page, click Extensions

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove).

Any improvement?

You are right on! Adobe Flash installed a theme add-on. I disabled it, and everything is back to normal. Thank you!

Hmm, I've never ever heard of Adobe doing that before. Just in case your Flash installer was an unofficial bundle that could contain less obvious things, please do check the Extensions as well.

Good idea. I looked, and there is nothing suspicious in the extensions. The only extension is DuckDuckGo. Maybe it's a weird interaction with that.