Please see VLC does not play (certain) videos under the Known Issues and Solutions section of the latest Update Announcement topic relevant to your system.
The item was first published in the 2025-07-12 Update Announcement thread – It is highly recommended that you always check these announcements for latest breaking issues that may apply to your system.
This is the relevant content:
This should arguably solve the issue for VLC, and as VLC also acts as a backend for other applications, likely Firefox too.
Naturally, let us know if this doesn’t solve the problem, and I’m sure a member of the community will offer to help when they are able.
In the meantime, please provide system information as described (below) to help others identify factors possibly contributing.
Regards.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit.
To that end, links are provided (below) - Please use them.
Be prepared to provide outputs from various commands when asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Waiting for others to blindly ask questions can be counter-productive – typically, nobody has a 
at their disposal – Instead, please help others to make informed suggestions, based on information you provide.
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
inxi --filter --verbosity=8
or the short form (preferred):
inxi -zv8
- inxi man pages (Arch Manual)