You can file web compatibility bugs on bugzilla.mozilla.org or webcompat.com

There are different ways how bugs are fixed. But someone might reach out to the page itself, find and fix a bug in Firefox or change the web specification if the incompatibility arises from ambiguity around the feature definition.

Firefox can also ship an intervention, basically injecting code into certain websites to fix broken ones.

Some incompatibilities can arise from missing features in Firefox, the web constantly evolves and the Devs sometimes don’t catch up. But bugs might still help, as high compatibility-risk features might be implemented more quickly.