Firefox includes MP3 among the "Open Media" types which have native/built-in support. This support was added in 2013, presumably partly because Mozilla determined that patents needed for MP3 decoding expired in 2012. (In some jurisdictions MP3 encoding remained patent-encumbered until 2017.)
Even though AAC-LC patents also expired in 2017, AAC is listed among "Patented media" and therefore only supported if 1) in a MP4 container and 2) the operating system provides support and Firefox manages to correctly use that support.
Red Hat's lawyers must have verified in 2017 that AAC-LC's patents had expired, because that year they shipped fdk-aac-free, a stripped-down AAC-LC-only variant of the Android fdk-aac encoder and decoder, in Fedora.
I was an early adopter and proponent of Opus. But because Apple has dragged its feet badly on Opus playback, AAC-LC is the defacto baseline audio format for the web. MP3 is too inferior and artifact-prone to serve as an adequate baseline. Mozilla's own MDN page about audio codecs, when discussing general audio playback, states "Based on this, AAC is likely your best choice if you can only support one audio format."
For me, whether FF has native support makes no difference as a user- FF detects and uses OS AAC support on all the platforms I use.
But as a non-professional web developer, it does make a difference. Very few of my sites' visitors are likely to encounter trouble: most people use Chrome or Safari, and most of the few that use Firefox are using Win10/11 or Android, and some others will have nonfree gstreamer plugins enabled. I feel guilty about not "doing my part to support the open web" by catering to FF users who don't have OS AAC support. But except for situations where Opus is markedly superior to AAC-LC (e.g. low bitrate speech) I just have a hard time justifying the resources involved in serving two copies of every audio file, since there are so few users who can't play AAC.
I recognize that detecting and refusing to try to play AAC files which are still patent-encumbered (notably xHE-AAC) takes effort since they aren't delivered with a different mime type etc, and properly informing users why such an AAC file won't play is also a concern. But we've reached a point where the few people who are using FF on the kinds of setups that don't support AAC are more likely to be understanding.
I believe the last HE-AAC (v1) decoding patents have also expired this year and that the last encoder related patents are expiring in a few months. So including HE-AAC v1 support may also be viable. HE-AAC v2 patents may last ~3 more years, while xHE-AAC will remain encumbered until ~2031. But most files on the Web are AAC-LC.