Setting all else aside, I'd use Firefox over Chrom{e|ium} browsers simply on technical and user-experience grounds. For general browsing, it simply works better, more quickly & smoothly.

Admittedly, I do keep Ungoogled Chromium & Brave around for the occasional Chrome-centric website, but I only end up using them a couple times a month.

I am only a causal Slack user, so while I haven't had any issues on Firefox using Slack, I could imagine there being issues.