Why is Firefox missing the border on some HTML tables

Maybe you've zoomed in/out a bit. This can happen either accidently or knowingly when you do Ctrl+Scrollwheel. Maybe it isn't completely resetted to zoom level zero. This mis-rendering behaviour is then recognizeable at some websites, also here at SO.

To fix this, just hit Ctrl+0 or do View > Zoom > Reset to reset the zoom level to default.

This is Firefox/Gecko bug 410959. This affects tables with border-collapse:collapse. It's from 2008 and there's no real progress on it, so you'll probably need to find a workaround. One way is using border-collapse:separate and fiddling with borders on a per-cell basis.

Building on the good answer of @GregL (I seem unable to "comment" directly on it): Instead of using JQuery to generate a "last" class, I simply used the built-in pseudo-element selector :first-child. I built rules selecting tr:first-child and td:first-child that define border-top and border-left (instead of border-right and border-bottom as in GregL's answer). The generic rules define border-right and border-bottom instead of border-left and border-top. :first-child is said to be supported in Chrome v. 4.0, Firefox v. 3.0, IE 7.0, Safari v. 3.1, and Opera v. 9.6 (). Tested on Firefox v. 40.0.3, where I saw this problem in the first place.

I found a similar problem when zoomed out in Firefox on an application I am working on.

The main cause was having the CSS border-collapse property set to collapse.

Changing it to separate instead fixed the problem. However, that meant I did have to rethink the way borders are applied to various parts of the table, because otherwise your borders will appear to double in thickness. I ended up having to use jQuery to give a special "last" class to the last td or th in each tr, and to the last tr in the table. My query was something like:

$('table tr > th:last-child, table > tbody > tr > td:last-child, table > tbody > tr:last-child').addClass('last');

My CSS rules were similar that:

table
{
    border-collapse: separate !important;
}

table tr, table th, table td
{
    border-right-width: 0;
    border-bottom-width: 0;
    border-left: 1px solid black;
    border-top: 1px solid black;
}

table td.last, table th.last 
{
    border-right: 1px solid black;
}

table tr.last td
{
    border-bottom: 1px solid black;
}

table
{
    border: 0;
}

I did end up using browser targeting so that I only applied these rules to Firefox users, but it may work for all browsers, I haven't tested.