1. speed: try using Google spreadsheet, it’s simply impossible. On Windows it works smoothly.
2. integration: try having to change your proxy settings times a day… FireFox can’t read proxy info from GNOME settings
3. UI: Save/Open dialogs are not the native GTK ones
4. crashes: hey where’s my firefox? it’s gone, without telling me anything… and this happens 3-4 times a day

I think Gnome FireFox needs more attention, anyway I’m waiting for Epiphany/WebKit and for a more Epiphany extensions.