Firefox: zoom changes page position

I use Firefox for Windows (XP, SP3). However, I suspect this behavior is 
the same in all Mozilla browsers that include a zoom feature.

Particularly in the middle of long pages, and especially with those that 
contain mostly text, changing either zoom setting (full or text) changes 
the relative vertical position of the page-- that is to say, the 
vertical portion of the page displayed in the browser window changes 
when the zoom setting is changed. With particularly long pages (book 
length, or even the length of a short story), a zoom setting change of 
only 5% will displace the vertical position of the page by several 
windows worth (even with Mozilla running full screen). Compare this 
behavior to that of Adobe Acrobat, which maintains the same page and 
relative vertical position on it when the zoom setting is changed. 
(Acrobat can be made to capture a web page as one [or sometimes several] 
long continuous page[s]. When displaying such PDF files, Acrobat is 
still able to maintain the relative page position when the zoom setting 
is changed.)

This behavior in Mozilla reduces the value of the zoom feature (although 
I suppose the situation could be worse-- I just discovered that IE 7 
repositions to the top of the page when the zoom is changed). It also 
reduces the value of 'Dog Ears'
(https://addons.mozilla.org/en-US/firefox/addon/4482
http://yellow5.us/firefox/dogears/),
a Firefox extension that allows the user to place index marks on pages 
and then seek back to them later (which provides a "bookmark" for 
specific content on long pages). Since 'Dog Ears' stores the absolute 
vertical position of index marks (probably distance from the top of the 
page), their positions relative to the page content changes if the zoom 
setting is changed. Many users asked the author about this problem in a 
forum (which I cannot seem to find, now), and he replied that the only 
other solution was to associate the index marks with content, which 
seemed unreliable.

My wish for the wish list, then: When a Mozilla browser is displaying 
somewhere in the middle of a web page, whatever is being displayed at 
the vertical center of the window will still be displayed there when the 
zoom setting is changed. Hopefully, whatever mechanism is developed to 
achieve that can also be made accessible to add-on authors, so that 
extensions like 'Dog Ears' are still useful when the zoom setting is 
changed.

That will probably be some work, and I wish I could contribute some 
code, but my experience is limited to writing shell scripts, and simple 
command line utilities in C. I hope I can persuade Mozilla developers to 
take it up, as the zoom feature is not really fully functional without it.

Thanks for listening,

Jeff