At the browser test, Qualys SSL Labs - Projects / SSL Client Test, you do not test Firefox RC4 fallback.
In version 38 (I think...) FF added an option of “security.tls.unrestricted_rc4_fallback” (a "True/False" option, I don't recall what is the default value).
This means that if you connect to a site with a RC4 cipher as its top cipher - you can decide if FF will fallback to RC4 ciphers and use them or not, when "not" means you will be blocked with a FF error.
I noticed that you browser check does not check this issue and so the user get a report that one's browser is not supporting RC4, while on the field it is eventually do - so users think they are safe by supporting only non-RC4 ciphers while at the wild they will not know their browser is transparently "falling" to RC4.
If you will sniff/wireshark a matching session you will see that the first "server hello" is failing the first "client hello" (sent with a list of ciphers that do not contain RC4 ciphers) and then you will see a new, second, "client hello", this time the cipher list will contain the same list as before plus 4 more ciphers, all supporting RC4 and all the about:config settings of these 4 ciphers is enabled by default at FF. This second attempt successfully establishes an RC4 based connection.
I believe SSL Labs should update its browser test to also examine this scenario so users will know their the real-life cipher support of their browser.
You can test this on the following sites, that support only two ciphers, both are only RC4 (and their matching SSL Labs server report):
You may have some issues with this FF setting, so please also see the following bug report I submitted to FF Bugzilla: