From MozillaZine Knowledge Base
HTTP is the application-layer protocol that most web pages are transferred with. This preference controls which version of HTTP to use if a proxy server is not configured.
Possible values and their effects
Use HTTP 0.9.
Use HTTP 1.0.
Use HTTP 1.1. (Default)
This preference should usually be left at the default of 1.1. 1.0 can be used if dealing with older server software, and will still work on the Internet at large. 0.9 will generally not work and is not recommended.
First checked in
Has an effect in