Firefox breaks Slack, Zoom, Salesforce, Jira, and several other internal/proprietary platforms I use. Many of our tools are integrated into each other (sometimes on the backend through the API, sometimes on the frontend through an iFrame), and Firefox really doesn't play nicely with these interactions. Either it doesn't like the fact that our apps are accessing multiple sites at a time and throws security errors, or it just doesn't render some parts of the page properly, making them unusable.
For instance, one ticketing tool we use is completely inaccessible in Firefox, because the page breaks after the header and loads the rest into a 10px-wide column that stretches for miles. Works fine in Chrome, Edge, and even Safari somehow.
Some of this could be fixed by using these platforms with their out-of-the-box software which may be more compatible with Firefox, without our modifications. But our mods are there because these integrations drastically improve our workflows, so that's unfortunately not a feasible option for our business.
A lot of this is due to Firefox having stricter standards, which isn't necessarily a bad thing. Maybe our developers should make our tools more standard-compliant and that might be better in the long run. But until then, I gotta use what works.