I've never seen a firewall option limited by the number of simultaneous connections, but the one and only way to find out for sure is to directly ask your hosting provider. Get it in writing, not just over the phone. And if you talk to them over the phone, ask the tech for his email address so you can send him a summary of the conversation and get a positive response from him that indeed, those topics and conclusions were discussed.
As for how fast is 10Mbps, let's do some simple maths:
- 1 megabit is 131,072 bytes
- Thus, 10 megabits is 1,310,720 bytes
- 1,310,720 bytes is 10,240 kilobits
- 10,240 kilobits is 1,280 kilobytes
- 1,280 kilobytes is 1.25 megabytes
Is 1.25 megabytes per second acceptable? Only you can answer that. If you have a site that has any amount of rich content, it's going to get saturated pretty fast. However, some reverse proxies and caching magic and you might be able to handle it pretty well.
The question remains, what kind of site and content are you going to be delivering? Build it and test what kind of kb load an average pageload costs and then extrapolate to logical conclusions.
In my completely arbitrary opinion based on no facts about your site, 10Mbps seems pretty slim.