In my experience there are almost never issues. Standardization has gotten much better, and as a web developer I appreciate all the effort the various browser teams have put into ensuring common behavior.
The effort of "supporting Firefox" as a developer is basically opening the app in Firefox and confirming that it works, which it basically always does because the app is built on standardized web features. (Other types of support might be different. e.g maybe you have help doc that tells your users how to clear the site cookies. Do you also want to write instructions for Firefox, Safari, Opera etc?)
The bigger concern is that some day Google might not need web standards. If developers target Chrome rather than the web platform, Google has free reign and no longer needs to compromise with Apple and Mozilla. For example, Mozilla's veto of the controversial Topics API would hold little weight if nobody cares about Firefox support. If Chrome gets to a dominant enough position it could move from embracing to extending quite quickly.
People have different opinions on Google, but even those who believe Google has no ill intentions still feel better if there are some checks and balances. Were Firefox to become obsolete, Apple would be the last one standing. And with the DMA, some are predicting that Safari could soon follow Firefox's decline.