There are these rules:

• line 33:
  chrome://browser/skin/places/editBookmarkOverlay.css

#editBookmarkPanelContent {
  min-width: 23em;
}

• line 2190:
  chrome://browser/skin/browser.css

/* Implements editBookmarkPanel resizing on folderTree un-collapse. */
#editBMPanel_folderTree {
  min-width: 27em;
}

Note that your System Details List shows multiple Flash plugins.

• Shockwave Flash 15.0 r0
• Shockwave Flash 14.0 r0
• Shockwave Flash 12.0 r0

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\

hey cor-el, so shouldve looked in browser.css after all, familiar with that one so mightve guessed the offending rule be there

ok so looks like the way to do it is using

   panel[type="arrow"] #editBookmarkPanelContent {
       max-width: 23em !important; }
   panel[type="arrow"] #editBMPanel_folderTree,
   panel[type="arrow"] #editBMPanel_tagsSelector {
       min-width: 18.2em !important;
       max-width: 18.2em !important; }

but does look better taking advantage of the whole width like

   panel[type="arrow"] #editBookmarkPanelContent {
       max-width: 23em !important; }
   panel[type="arrow"] #editBMPanel_folderTree,
   panel[type="arrow"] #editBMPanel_tagsSelector {
       min-width: 22.3em !important;
       max-width: 22.3em !important;
       margin-left: -3.4em !important; }
   panel[type="arrow"] #editBMPanel_newFolderBox{
      -moz-margin-start: 12.3em !important;}

or made closer to the default width when expanded

   panel[type="arrow"] #editBookmarkPanelContent {
       min-width: 27em !important;
       max-width: 27em !important;  }
   panel[type="arrow"] #editBMPanel_folderTree,
   panel[type="arrow"] #editBMPanel_tagsSelector {
       min-width: 26.3em !important;
       max-width: 26.3em !important;
       margin-left: -3.4em !important; }
   panel[type="arrow"] #editBMPanel_newFolderBox{
      -moz-margin-start: 16.3em !important;}

the panel[type="arrow"] prefix is only needed if its part of a global style using @namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul); instead of @-moz-document url("chrome://browser/content/browser.xul") { } because gatekeeper will apply it to bookmarks library aswell, despite the element names suggesting theyre specific to the toolbar panel theyre also used in the library. Also the expanded width snippets move the new folder button to the right cause its only partly responsive on the far left having only half the button clickable

anyway thanks for pointing the right direction, and pointing out the multiple flash plugins, cheers