Your connection is not secure
Firefox 49 says; "The owner of .. has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website. Actually tis is a government site. Almost all helpers advise "enter about:config" then find change "security.enterprise_roots.enabled" from the list opened, and then chance the value from "false" to "true". But, there is no "security.enterprise_roots.enabled" in the list ???
Wšě wotmołwy (2)
I expect you know you're using a version of Firefox that's roughly 7 years old? "security.enterprise_roots.enabled" does appear as an about:config option in recent releases, and has been available since roughly 2018. While I do think upgrading Firefox will solve this, suggestions to use "security.enterprise_roots.enabled" could be misleading. This option seems to exist mostly to work around antivirus software that doesn't acknowledge that Firefox uses its own certificate store. Do you think AV software is a contributing factor? My suggestions are
1) Upgrade Firefox. Then you can try "security.enterprise_roots.enabled".
2) If for some reason you can't, see if anything at https://support.mozilla.org/en-US/kb/error-codes-secure-websites helps.
3) Post more details. Does your error look just like the one on the above web page? Could you post a window grab of the error and a grab of the actual certificate? Feel free to blot out any text that may be private. Does this problem happen on any other web sites? What AV software are you using?
It is more likely that the website only supports modern cipher suites and possibly TLS versions that Firefox 49 doesn't know of and using the current release (119.0 or 115.4.0 ESR) will likely fix this.