I don't think that page is accessible unless you have an account.

Have you noticed any background colors missing on other pages, or just this site?

In case there was a site change but Firefox is not fully recognizing it, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache (only select "Temporary cached files and pages", don't clear other kinds of data)

If you have a large hard drive, this might take a few minutes.

Then try reloading the Garmin page. Does that help?

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page and signing in again. Does that help?

I did all of that, but the menu of this site has been having problems since the end of April. Where can I write to the browser developers? I hate bots, I could find an answer like this on the internet without creating an account and writing here.

Hi

Have you considered updating to the more recent version of Firefox ESR?

I did all of that, but the menu of this site has been having problems since the end of April. Where can I write to the browser developers? I hate bots, I could find an answer like this on the internet without creating an account and writing here.

I'm no bot, and you didn't answer my question.

Assuming all other site colors are normal and only this page is affected:

Firefox 115 is on Extended Support Release, so it only gets light security fixes. You can ask Garmin to look at the ones in the April 29th release to see whether one of those could have affected the menu background color: https://www.mozilla.org/firefox/115.23.0/releasenotes/

Otherwise, maybe another Firefox user that has a Garmin account can examine the page and come up with a workaround.

In case you haven't already ruled out settings changes and add-ons, here's a quick way to test:

New Profile Test

This takes about 3 minutes, plus the time to test Garmin.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2025, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2025 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

Is the menu still missing its background color in the new profile while other sites look normal?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2025 will remain available for future testing.)

And yes, that is a templated answer ;-)