It's likely that a recently-released new feature in Firefox, or a recently-installed extension, now uses the ALT+C shortcut.

Support requests and bug reports should be posted to the AE Creations Help and Support forums: https://groups.io/g/aecreations-help

According to Google Translate, you wrote that Clippings Manager keeps minimizing itself on Ubuntu.

In Clippings Manager, click Tools on the toolbar and then select Minimize When Inactive to toggle the setting on or off. This setting exists in Linux only and is a workaround for the window always being on top of other Firefox windows. You can opt to turn the setting back on when you need to switch between windows so that it doesn't get in the way.

In an upcoming new version of Clippings, an icon on the Clippings Manager status bar will make it easier to quickly turn the setting on or off in just one click.