SUMMARY

When using Firefox on KDE Plasma to access some sites (e.g., WhatsApp Web and the Rich Text Editor on Reddit), composing a word that begins with a dead key accent (e.g., ´ + a = "á" to type "árbol") causes the accented first letter to disappear. Dead keys work correctly when not used as the first character (e.g., in "información"). Switching KDE's Virtual Keyboard from "Plasma Keyboard" to "IBus" fixes the issue.

STEPS TO REPRODUCE
1. Set "Plasma Keyboard" as the input method in KDE System Settings > Keyboard > Virtual Keyboard.
2. Open Firefox and navigate to WhatsApp Web (web.whatsapp.com) or Reddit (reddit.com).
3. Write "árbol" and "información" in any chat input if testing on WhatsApp Web, or in the input box in the Rich Text Editor if testing on Reddit.

OBSERVED RESULT
While writing "árbol", the "á" key disappears. Everything works as intended while writing "información".


EXPECTED RESULT
Both words should be displayed correctly without disappearing.


SOFTWARE/OS VERSIONS
Linux: Fedora Linux 43 (also tested on Fedora Linux 44 and CachyOS) 
KDE Plasma Version: 6.6.3
KDE Frameworks Version: 6.24.0
Qt Version: 6.10.2

ADDITIONAL INFORMATION
- Keymap is set to "latam".
- Discussed on Fedora Discussions: https://discussion.fedoraproject.org/t/dead-key-input-fails-on-first-character-on-firefox/183565
- Some tests: https://discussion.fedoraproject.org/t/dead-key-input-fails-on-first-character-on-firefox/183565/4
- Related issue in Mozilla's Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=2004150
- I cannot reproduce this issue on Chromium (both with IBus Wayland and Plasma Keyboard) and on Firefox using IBus Wayland.