Some bug reports about weak cipher suites:

• bug 917047 - Remove the security.enable_md5_signatures pref
• bug 650355 - Stop accepting MD5 as a hash algorithm in signatures (toggle security.enable_md5_signatures to false)
• bug 799007 - Remove support for low/weak/null cipher suites

Thank you for links. There is not answer of questions about Firefox 28 version and it's MD5 certificates support now and in future.

If MD5 will not be supported, this explain why option is missing, but not why option is present in some installations.

If MD5 is still supported, this mean that there is a bugs: missing option in some installations (different 28 versions?) and not working function with MD5 certificates in 28 version.

If prefs appear user set (bold) on the about:config page then they won't be removed automatically and you need to reset such a pref and close and restart Firefox to remove such a no longer supported pref.
Only default prefs that haven't been modified and have the default value are removed from the about:config page if they areno longer supported.

Thank you very much for useful explanation.

Finally, from removing of "security.enable_md5_signatures" option, we must understand that MD5 certificates will not be supported from Firefox 28 and future versions, even option is set "true" in user set after updating the browser?

We are familiar that from Firefox 16 there is no longer supporting MD5 as a hash algorithm in digital signatures, but if "security.enable_md5_signatures" option is set "true", browser works with them. May be this was changed here and MD5 support is stopped?