Updated: May 30, 2025
Yows. What I'm going to discuss today ain't a new problem. I've mentioned it at least 5-6 times, and it seems to affect multiple versions of Plasma, including various 5.X and 6.X branches. I've seen the problem in several distributions, albeit all Ubuntu-based. What happens is, you update your software, and suddenly, the icons no longer show up, either in the menu, or in the task manager, mostly the former.
In particular, Firefox seems most prone to exhibit this problem, and I've seen it lots of times. You run an update, and the icon is no longer there. You have an empty placeholder. The program works normally, and you can launch it, even by clicking on this supposedly generic non-icon icon. Well, let's troubleshoot and resolve the problem, shall we.
Problem in more detail
As you can see above, there ain't no Firefox icon in the menu. The task manager shows it just fine. The program works without any issues. I don't have a definite proof, but this is the sequence of events that leads to the potential manifestation of the issue:
- Your system comes with a snap version of the browser. During one of the updates, something goes wrong.
- In parallel, you may have set up another version of Firefox, like say from Mozilla's official tarball. The program will correctly show with its icon in the menu for a while (it will use the snap's icons, actually), but then, during an update, that icon will somehow disappear and affect BOTH programs.
- But ... but ... I also noticed this on a distro that uses Firefox deb, and no snap. The plot thickens.
For whatever reason, the icon is gone. Indeed, you can check this. Right-click on either one of the two entries, select Edit Application. It will now show you the properties windows for the application launcher, called [program name].desktop. In my case, it's firefox_firefox.desktop. Now, click on the empty icon area to open a window that lets you choose a new icon for the program. Here, click on the dropdown box (which shows Applications), and select All, so you can search in all available categories.
Now, type firefox in the search box, and it won't show up, at all. It's gone. Completely. The reason for WHY it disappeared is up to the Kubuntu, KDE neon, Plasma, maybe Ubuntu and snap folks, maybe Mozilla to figure out on their own. I'm going to show you a neat workaround.
Solution
Quite simple. Download the desired program icon - in this case, Firefox. For example, you can grab it on Wikipedia (svg, so you resize it to your heart's desire, and will obey any resolution or scaling), or maybe get it from Mozilla. You can also choose png files (ideally with transparency), or any which icon you like.
Go back to Edit Application > Select Icon. Click Browse. And select the icon you want. You can do the same by editing the relevant program entry through the Plasma menu. You will need to do this separately for each installed program version and/or menu entry. We've talked about this in the past quite some, especially if you do actually want to maintain multiple versions of certain programs on your machine, have separate launchers for them, and be able to distinguish among them easily.
And problem solved!
A tiny alternative ...
Now, if for some reason you don't feel like manually fiddling with icons, and would like to tell the system to "fix" itself, you could simply tell the Plasma desktop to try to rebuild its caches and indexes. Now, this won't work if the icons are truly missing, as I've shown above, but you might as well run a couple of commands, log out and then log back in to see if this sorts out your problem.
sudo update-mime-database /usr/share/mime
sudo update-desktop-database
Conclusion
Here we go. Your Plasma desktop is now pristine again, and the OCD demons may rest once more, until the next issue. I'm not 100% certain why there ought to be any issues with icons, but if the problem happens, you ought to be able to resolve it quickly. After all, missing icons are an eyesore, even if your aesthetic levels and demands ain't that high.
In my case, I believe the problem is a manifestation of a botched update that removed the necessary program icon, which then triggered the issue for all the different versions of Firefox installed. Luckily, in Plasma, it's quite easy to customize program launches and assign them new icons. On one hand, we have an issue that shouldn't have happened. On the other, the Plasma desktop is amazing and flexible, and you can remedy problems, whichever they may be. So, yeah. That would be all, fellas.
Cheers.