When I last discussed SSI the other day I was waiting on IBM.
It seems that they were able to validate that Firefox did not work with the SSI since 6.5 of Domino. Granted this may not affect many people and the info on the web is sketchy but here in better detail is what I was told, thanks to Jason Bean of IBM Support:
We have found the problem and created a fix to resolve this issue! After digging into our code some more, we discovered that when a SSI directive is triggered while rendering, a sub request is spawned for the resource referenced in the include. The creation of this sub request was inadvertently dropping the first header listed in the GET, which in the case of Firefox is the "host" header.
The result is that Domino is not able to map the sub request to the appropriate web site (since we don't know what hostname it is for), causing the include directive not to be processed. This also explains why making the site a "Default Site" or placing a copy of the includes in the default html directory resolved the issue.
I was asked yesterday to make the site with the problems the default site in my internet documents. Which I did and it worked fine, but was not a great solution as we did not want that site to be the primary one.
Imagine my shock this morning when I got an email saying a hotfix was not only tested but approved for usage. Less than a day after it being submitted. Kudos to the development team at Iris or the IBM labs who did it. A small little piece like this to be done was impressive to me and the client. Thank you again.
Oh, if you need this fix, ask support for it (hopefully it will make it into 8.5.2) or email me if you need it as I can't post the ftp for it.
The name is 851HF360_w32.exe and is a small little thing at a few 100K.