Are you referring to this?
What exactly is the issue? Is the dialog taller than the height of the browser viewport and the bottom part with the apply button is hidden below the bottom of the browser window? (that's what you mean by "not resizing correctly"?)
I can't replicate any problem in Firefox 52 under Win XP (that's the newest version that can be used on this OS), so can't see what you see but can guess based on what I see.
FireFox Web Developer Tools Style Editor
You can do interactive things to the dialog via CSS using the built in Firefox style editor. To get to the style editor, Hit Shift-Ctrl-K, then click the "style editor tab"
Have the search page open with the problematic cut off refinements dialog open.
Hit the "new" button on the left side of the style editor to create a new temporary stylesheet.
On the right side, type (or paste) the following in as line 1: div.wnd.sa {margin-top: 100px;}
that should shove the dialog down 100 pixels (which isn't what we want, but demonstrates the idea)
Change the 100px to -75px and the dialog should slide up (maybe too much and cut off at the top, or maybe not enough and apply button is still hidden) Play with the numbers and see what works.
One other thing that might do the trick is to zoom only the dialog (when you do the browser zoom, it zooms everything. Here is how to zoom just the dialog:
div.wnd.sa { -moz-document: scale (0.9); }
play with the zoom factor to see if that does the trick.
You can apply multiple rules like this:
div.wnd.sa { margin-top: 50px; -moz-transform:scale (0.8); }
or like this
div.wnd.sa { margin-top:-100px; }
div.wnd.sa { -moz-transform: scale (0.7); }
or like this
div.wnd.sa
{
margin-top: 50px;
-moz-transform:scale (0.8);
}
That sort of works, scaling or moving it around, but this may be a better way to do it. We can try to just reduce the dialog height by brute force.
(first: you might want to comment out the previous rule(s) by just prefixing the with an XX (or pretty much anything) like this:
XXdiv.wnd.sa { margin-top:-100px; }
XXdiv.wnd.sa { -moz-transform: scale (0.7); }
which causes the div.wnd.sa selector(s) to not match anything on the web page (because it is very, very unlikely there are any 'XXdiv elements with wnd and sa class attributes), so the actions (margin-top or -moz-transform) won't apply.
Add this rule after the other ones:
div.wnd.sa>div.wnd-f>div.wnd-b>form#see-all-form>table#see-all-dialog
{ height:375px !important; }
That reduces the dialog I see (which isn't bigger than the browser window to start with) by about 20% in height. If that does the trick, woohoo. Adjust values as needed, use multiple rules if needed (reduce the height, move it up or down etc) Likely that rule will be all that is needed.
I don't use the style editor myself and have no idea how to make it a permanent fix using this approach. (the above was just sort of (bad) tutorial on a method. You might have to go through the procedure every time you need to adjust filters 
(not the whole procedure, just pasting in the rule(s) you decide do the job into the style editor which would probably be just the last one.)
The style editor can save the new stylesheet to disk, it's just that Firefox can't autoload it next time the browser starts. The saved stylesheet can be loaded using import in the style editor, but that whole approach is really klunky.
There is another way to do so though.
Usercontent.css
You locate (or create with a text editor) a file called usercontent.css which is a Firefox provision for user supplied CSS rules.
Answer 2 here demonstrates it well: https://superuser.com/questions/318912/how-to-override-the-css-of-a-site-in-firefox-with-usercontent...
Paste the following in the file (instead of the example code the poster on superuser gave):
@-moz-document url-prefix('https://www.ebay.com/sch/") {
div.wnd.sa>div.wnd-f>div.wnd-b>form#see-all-form>table#see-all-dialog
{height:375px !important;}(of course, you want to change that 375 to whatever you worked out using the style editor method)
Once that content is saved in usercontent.css, it will load every time Firefox starts, and will apply only to eBay search pages (the url-prefix) and will further only apply to page elements on those search pages that match the fairly specific
"div.wnd.sa>div.wnd-f>div.wnd-b>form#see-all-form>table#see-all-dialog" selector chain.
The drawback to the usercontent.css approach is that to make any changes to it you have to edit it and restart Firefox. Not very convenient for tinkering but ok if you set and forget.
Stylish Addon
How would I do it? I use a Firefox addon called Stylish. Same idea as the above two approaches, but has an editor, GUI, and is great for tinkering (I run a little over 3000 lines of CSS rules in Firefox just for eBay to beat it into a form I like).
To implement the fix using Stylish, you would create a new Stylish rule (after installing Stylish), and use exactly the same code as in the codebox above for usercontent.css
Bookmarklet
There is a fourth way to do it - using a Firefox bookmarklet. The bookmarklet would be a bookmark on your browser toolbar containing JavaScript that applies the same CSS rule(s). You would just click the bookmarklet every time you run into the misbehaving dialog and it would resize it.
javaXscript:(function(){
var newWinHeight='375px';
if(location.href.match(/^https:\/\/www\.ebay\.(com|ca|co\.uk|com\.au|in|de|at|fr|
it|nl|es|ch|ie|ph|pl|be|com\.hk|com\.my|com\.sg)\/sch\//i)==null) return;
var formElem=document.getElementById('see-all-form');
if(formElem!=null) if(formElem.tagName=='FORM'){
if(formElem.parentElement.parentElement.parentElement.style.visibility!='visible') return;
tableElem=document.getElementById('see-all-dialog');
if(tableElem!=null) if(tableElem.tagName=='TABLE')
tableElem.style.setProperty('height',newWinHeight,'important');}})();
Copy the above, paste into the location field of a new browser bookmark, remove the X in javaXscript (placed there to get around forum software reformatting), name the bookmarklet whatever you want (SearchRefResize), and good to go. If the resizing isn't right. edit the properties of the bookmarklet and adjust the newWinHeight value as needed.
JavaScript
A fifth way is the same JavaScript approach as the bookmarklet, but implemented as a GreaseMonkey script (which requires installing the GreaseMonkey addon and is the most complex approach)
There are also other Firefox addons (that I don't use and haven''t tried to apply CSS rules).
If you want to try any of this and need help, just ask.
(or maybe someone will pipe in with an obvious simple solution I didn't see 
)
