> I keep seeing this argument against using Firefox, but as a long time user I fail to see any glaring issues with it.

No glaring or usability issues.

What happened is that Firefox added some defaults that mimic a tiny bit of Chromium browser behavior.

    Recommend extensions as you browse
    Recommend features as you browse
    Send technical and interaction data to Mozilla
    Allow websites to perform privacy-preserving ad measurement
    

There's that and the long-time sponsored crap on the new tab page. It takes a moment to toggle it all off.