I believe some contexts and/or CSS attribute combinations make “table-cell” display type unusable; I might have seen something about it at the time of posting.
If you have a fragment of code where “table-cell” doesn’t work – please post it here.

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If we remove everything unrelated to positioning (in FF3 only), we are left with the following CSS code:

.nav_next a {
display: table-cell;
width: 85px;
text-align: center;
margin-top: -60px;
padding-top: 60px;
top: 60px;
}

{ “height” is omitted because of the margin/padding/top attributes of 60px, which renders height=16px irrelevant (at least in FF3); “width”, however, influences the display, so it is left in. “vertical align” also had no effect in FF3, and should have no effect in IE6, so I removed it. }

Let’s have a look at what is going on.

By applying display:table-cell, you make the anchor element behave like a table-cell. This, in turn, enforces proper width of the element, and makes all of margin/padding/top attributes exhibit desirable effects in FF3.

An equivalent, which works just the same in FF3, is to set display:block and add position:relative (which isn’t necessary for FF3 with display:table-cell).

Applying padding-top:60px pushes the anchor’s text to the bottom, and text-align:center centers text. Negative margin-top:-60px looks like elements flow fix (had no effect in my test file with only two elements; might also be IE6-specific).

So the minimal working version might be:

.nav_next a {
display: table-cell;
width: 85px;
text-align: center;
padding-top: 60px;
position: relative;
top: 60px;
margin-top: -60px;
}

with the last three attributes unnecessary for FF3.

Let’s label this solution as “display:table-cell extended”