If anyone is interested in that:

I fixed the irregular display with Firefox using <div class="noflow"></div> inserted at the end of the entry-text
and a class .noflow  {clear: both;} in the stylesheet.

But as my fix is for a considerably modified 1.7.5, i can't point out the changes for 2.0.

For 1.7.5 You'll find it in

• posting.php
  
• board_entry.php
  
• forum_entry.php and
  
• mix_entry.php

Look for <div class="postingboard"> and <div class="posting"> and change the corresponding lines inserting
<div class="noflow"></div> as in

echo '<div class="postingboard">'.$ftext.'<div class="noflow"></div></div>'; (board_entry.php, 2 occ.)

?><div class="posting"><?php echo $ftext; ?><div class="noflow"></div></div><?php (forum_entry.php)

echo '<div class="postingboard">'.$ftext.'<div class="noflow"></div></div>'; (mix_entry.php)

echo '<div class="posting">'.$pr_text.'<div class="noflow"></div></div>';
and
echo '<div class="postingboard">'.$pr_text.'<div class="noflow"></div></div>'; (posting.php)

The last </div> may be a </p> in Your code, as I've changed that because of validation problems due to other modifications.

Then using [img=left] or [img=right] the signature will allways be at the end of the post, or with no sig. the gap between post and bottom will be normal and the overflow of the image into the edit area won't occur.