Firefox is much stricter in parsing comment tags than Safari or IE.
Comments should have 2 hyphens after the exclamation point, no more, no less, and there should be no hyphens within the body of the comment.

So:

<!--

is OK, but

<!----

is not, particularly in Firefox.

Example 1 shows a table surrounded by a properly formed comment tag.
Example 2 shows the same table surrounded by a tag with too many hyphens - the table and all code between comment tags disappears - in Firefox.

View the code source in Firefox to see how firefox colorizes the code:

With properly formed comment tag:

Improperly formed tag: