Been using this service for about a year now, and just found out that they have a Firefox addon.

Reasons I like the service:
- Not ran by a tech giant.
- The servers used to host the service are not by a tech giant.
- The the pricing is good, and cheaper than other services.
- Has the right mix of custom settings, and easy use.
- They are fast to support emails.
- it is open source.
- and the default domains are not blacklisted on any sites I know of.

Also I have tried, and used many other services before, and by far this is the best one.