Also, the Tor network probably doesn't have enough exit node capacity to handle the volume it would get.
This is no different from your ISP or the guy next to you on the coffee shop wireless.
If you don't want people to snoop and rewrite your HTTP connection, don't use HTTP. And the solution to broken HTTPS is to fix HTTPS. Tor doesn't decrease your security in either of these cases.
> That's a risk you take when using Tor, and it's not a tradeoff you can reasonably convey to existing Firefox users who are used to Private mode meaning a certain thing.
1. It's not a tradeoff. Tor does not decrease your security in either case. There are cases where Tor doesn't provide privacy protections, but there's not a case I know of where using Tor provides fewer protections than not using Tor.
2. It's impossible to convey to non-technical users that private browsing doesn't provide privacy from their ISP, governments, etc. I don't understand how you can claim that providing fewer privacy protections is less confusing to the non-technical user.
> Also, the Tor network probably doesn't have enough exit node capacity to handle the volume it would get.
This is a solvable problem.
You're a paying customer of your ISP and they're bound by the law and your contract.
Tor exit nodes can be anyone, anywhere, who feels like messing up your traffic.
For example, by what definition would this not be 'safe'? Do you mean in comparison to Firefox's defaults, which enable profoundly privacy-invasive tracking, hide the contents of the cookie management dialog box, and enable the delivery of malvertising at will by visiting "safe" websites - just to name a few?
MitM positions exist on the internet for all users today, both for HTTP and in the situation of "new HTTPS bugs". So you point out risks taken when using the internet in general, and the Tor Browser Bundle already mitigates many of those risks via NoScript and various patches. I've used Tor Browser Bundle daily for all purposes - including online banking and shopping - for several years with zero problems. Malicious exits exist and get flagged out of the network rapidly. Essentially all remaining risk is trivially defeated by toggling the 'block all unencrypted requests' pref in HTTPSEverywhere, which is part of the Tor Browser Bundle and could be built into an implementation in Firefox with a warning for HTTP traffic.
And you're probably thinking of 2009-era Tor network experience. For me, Tor network performance routinely boils down to ~200ms additional latency with ~1.5-2MB/s download speeds. The difference between that and an average broadband connection is barely noticeable.
I recommend using the Tor network and the Tor Browser Bundle before criticizing this idea.