Firefox keyboard input delayed by 17 seconds on Ubuntu 24.04 with KDE and X11

I'm running Firefox on Ubuntu 24.04 with the KDE desktop environment under X11 (not Wayland). Firefox works normally for several days without issues. However, suddenly, it stops responding to keyboard input in a peculiar way: keystrokes are registered, but they are delayed by 17 seconds before appearing. This delay only affects Firefox - other applications like Konsole, gedit, Brave, and VLC respond to keyboard input normally without any lag. These are affected: * Address field * Copies in Print dialog * Text field in Gmail These are not affected: * Page-Up/Down in scrolling * CTRL-Page-Up/Down (change tab) * CTRL-P (open print dialog) * Esc (Cancel print dialog) The system is otherwise stable, and restarting Firefox resolves the issue temporarily. The problem may reappear hours or days later, seemingly at random. I haven’t noticed any specific triggers yet. ibus restart does not fix it. Has anyone experienced this issue or have insights into what might cause Firefox to exhibit this delayed keyboard input behavior under these conditions? Are there logs or debugging steps I could use to narrow down the cause? OS: Ubuntu 24.04 DE: KDE (using X11, not Wayland) Firefox: Mozilla Firefox 135.0.1