https://github.com/jenkinsci/jenkins/pull/8753

I can't duplicate that behavior with Firefox 120.0 on my Windows 11 computer. Can you provide more details to describe the configuration where you see that behavior?

I am using Windows 10, with no special configuration (style customizing plugins or similar) in Firefox. Most of my colleagues do not have this problem, although there is one person who is also affected. I'm not really sure why it works at all, since there is no background attribute set.

Computed styles show no background style:

So the default browser style applies: rgba(0,0,0,0) = transparent

The inspected node is part of this dropdown:

Can you check what is the setting of 'layout.css.backdrop-filter.enabled' in your firefox?

If I change this to false (default is true) then I also see this. Changing this requires a restart of firefox to take effect

the backdrop-filter is defined on 'div class="tippy-box"'

layout.css.backdrop-filter.enabled is set to true

https://developer.mozilla.org/en-US/docs/Web/CSS/backdrop-filter says it is not supported in my browser, I can try to find out why that is the case.

Setting layout.css.backdrop-filter.force-enabled to true works:

Not sure why this is necessary though

According to https://stackoverflow.com/questions/63789769/why-backdrop-filter-in-firefox-dont-work there is another property to check: gfx.webrender.all

Thanks for the help. How should we proceed here?

I tried setting gfx.webrender.all to true, but that alone was not enough - I still needed layout.css.backdrop-filter.force-enabled

As I have now a working solution for this seemingly very rare problem, I will close the issue.

lgf10 Is there something unusual about your client graphics hardware or OS?

I tried setting gfx.webrender.all to true

References I found mentioning this also mentioned the need to restart the browser for the change to be effective, did you?

I just did some digging in about:support and while I'm not sure if I would call it unusual, it explains everything.
I am using Remote Desktop, which results in the real GPU being replaced:

This causes Firefox to disable the backdrop-filter, since it cant parse the driver

I guess we have now reached the end of the rabbit hole. I would never have guessed that the dropdown being transparent would in any way be related to Remote Desktop - I don't think I have ever had anything remotely similar to this issue. Again, thanks for helping solve this issue