First, you should never install a Certificate Authority (CA) file unless you thorougly trust who generated that file. Doing so tells your email program or browser that you trust whoever generated it to tell you that the secure sites you are connecting to really are the actual, real sites you are connecting to, and not something faked.
As such, CA files are powerful. Your web browsers, email programs and operating systems usually come with a bunch of CA files already pre-installed from companies that are supposedly trustworthy enough to that software maker.
In the case of HGS CA certificates - we generate many SSL encryption certificates for our devices and for our clients who wish to use them internally, instead of purchasing 3rd party encryption certificates. But we do not pay large sums of money to other companies to get their "rubber stamp" of approval on our certificate - so email clients and web browsers will pop up a warning to users saying that you are connecting to a site with a bad SSL encryption certificate and will sometimes force you to make an exception before you are allowed to connect.
However, by installing the Home Grown Systems Certificate Authority (CA) file yourself, you are telling your program that you will trust SSL encryption certificates that were generated by Home Grown Systems, SPC - and so you will not get the scary warnings any more when using our encryption certificates.
Installing this certicate is better than just accepting an exception forever, because if you accept an exception forever you might be spoofed. However, if you install the HGS CA file, you will continue to get assurance that what you are connecting to is valid, according to all the CA authorities installed, which now includes your HGS certificate authority.
First you must download the HGS Certificate Authority File. Right-click and select Save As... and remember to which directory you saved it.
If you want to install it in your web browser, just click on the file instead of selecting Save As... and you can then tell your browser to trust it. (if you are using Firefox).
Go to Options in your Thunderbird menu, which will bring up the screen shown.
In this screen, hit Advanced, and select the Certificates tab.
Then hit the big View Certificates button.
After you hit View Certificates make sure you are in the Authorities tab.
Hit the Import button and select the HGS_CA_Public.crt file you downloaded.
After you hit the Import button and select the HGS Certificate Authority File you downloaded, you will be presented with a trust dialogue.
After you select the trust boxes you'd like, hit OK on out of everything.
If all goes well, you should no longer receive warnings about ceritifcates from Home Grown Systems, SPC.