Many public Internet websites filter connections from the Tor network, many other websites are very slow, yet others impose extra obstacles such as multiple rounds of captchas (even 5 or more) or degraded service (including high suspicion of payments), and of course you often will receive webpages in the wrong locale or language - which can trigger regional filters. Currently, workarounds requires resetting the circuit (few non-technical users will even understand what the circuit is), lots of patience and reloads, and often just giving up. [EDIT: And non-technical users won't understand what is happening and therefore won't know when to use which workaround.]
If that's the experience of typical Firefox users, they won't use it and they will have bad associations with Tor and Firefox.