Firefox not recognised, shown as "unknown"

- [x] I am on the [latest](https://github.com/ActivityWatch/activitywatch/releases/latest) ActivityWatch version.
- [x] I have searched the issues of this repo and believe that this is not a duplicate.

- **OS name and version**: LInux, Fedora Workstation 39, KDE on X11
- **ActivityWatch version**: v0.12.2

## Describe the bug

In the "Activity" view in the web UI, all time spent in Firefox is only shown as "unknown", both in windows and in applications.

## To Reproduce
1. Install current ActivityWatch version (v0.12.2) on Fedora 39 with KDE on X11
2. Start ActivityWatch
3. Open Firefox and browse for some time
4. Open the ActivityWatch WebUI and click on "Activity" in the top left corner
5. Top Window titles and Top Applications show "unknown" instead of "firefox"

## Expected behavior

Firefox is recognised as application and activities are assigned according to category rules.