This issue is still open.

What I have implemented to fix this problem is to remove ALL occurences of TABINDEX so that order defaults to display order. In this manner the tab order works perfectly for the custom fields. This has to be done on
bug_report_advanced_page.php
bug_report_page.php

Example
<select tabindex="1" name="category">
is changed to
<select name="category">

Hopefully this will get picked up and implemented

This also worked for me this needs to get included in the next release.

A few issues have been raised on this but it has not been solved properly I believe. Here is my implementation.

1) Add Function to the helper_api
function helper_get_tabindex() {
static $t_index = 1;

    $t_index = $t_index + 1;

    return "tabindex=\"$t_index\"";
}

2) And in the custom_field_api

function print_custom_field_input( $p_field_def, $p_bug_id = null ) {
$t_tabindex = helper_get_tabindex();
etc...

And add tabindex property to all html echo statements, for example

echo '<select name="customfield' . $t_id . '[]"' . $t_tabindex . ' size="' . $t_list_size . '" multiple>';

3) The bug_report_page.php should also be changed to replace all hard coded tabindex references with a call to this new helper function. For example for the category field which is hardcoded to be the first one (and which in all likelihook it will remain :-> ):

<select <?php echo helper_get_tabindex() ?> name="category">
?php print_category_option_list( $f_category ) ?>
</select>