That rule still looks to be correct in Firefox 26. Is that the first rule in the file after the namespace line? (Just wondering whether an earlier rule might be corrupted in some manner that is preventing Firefox from processing the file correctly.)

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

Does other code in the userChrome.css file still work?

Yes, the other code does work. There is still no close button on the addon bar, no tab scroll buttons, certain dialog boxes are still bigger, the fonts are still bigger, the background behind the security icon for mixed content is still red, etc.

I tried moving that command to the end of the items in the file and the star is now gone again. What could have overridden it, and why did that happen just recently?

 I did not notice earlier when I first installed that version of Firefox because I have been using SeaMonkey most of the time.

Hi finitarry, it's hard to answer questions about cascade precedence or potential syntax issues without seeing the file. But, glad to hear it is working again.

You can post the (full) content of the userChrome.css file in case you want us to check the content for conflicting rules or post the file temporarily on pastebin http://pastebin.mozilla.org if the file is large.

/ * Do not remove the @namespace line -- it's required for correct functioning */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
/*
 * For some examples see http://www.mozilla.org/unix/customizing.html
 */
/* Bookmarks toolbar */
toolbarbutton.bookmark-item, 
.bookmark-item > .toolbarbutton-icon,
.bookmark-item > .toolbarbutton-text {
margin: 0px 0px 0px 0px !important;
padding: 0px 0px 0px 0px !important;
border: 0px !important;}

/* Navigation toolbar */
#back-button .toolbarbutton-menubutton-dropmarker,
#forward-button .toolbarbutton-menubutton-dropmarker {
  display: none !important;
}
#unified-back-forward-button,
#unified-back-forward-button * {
   border: 0px !important;
}

#back-button {
  -moz-appearance: none;
  padding: 0px !important;
  margin: 0px;
  border: none !important;
}
#forward-button {
  -moz-appearance: none;
  padding: 0px !important;
  margin: 0px;
  border: none !important;
}

/* Other toolbars */
.toolbarbutton-icon {
padding: 1px 0px 1px 0px !important;
margin: 0px 0px 0px 0px !important;
border: 0px !important;}

.toolbarbutton-menubutton-dropmarker {
 padding: 2px 0px 0px 0px !important;
-moz-padding-end: 0px !important;
-moz-padding-start: 0px !important;
margin: 0px 0px 0px 0px !important;
border: none !important;}

/* Menus */
#context-sendpage {display: none !important;}
#context-sendimage {display: none !important;}
#context-sendlink {display: none !important;}

#subscribeToPageMenuitem, #subscribeToPageMenupopup {
display: none !important;}

#BMB_subscribeToPageMenuitem, #BMB_subscribeToPageMenupopup
{ display: none !important; }
#BMB_unsortedBookmarks,
#BMB_viewBookmarksToolbar, #BMB_viewBookmarksToolbar+menuseparator
 {display: none !important;}

menu.bookmark-item,
menuitem.bookmark-item {
  max-width: 48em !important;
}

/* Increase size/height of the expanded bookmarks panel */
#editBMPanel_folderTree,
#editBookmarkPanel  {min-width:350px !important; min-height:350px !important;}

#search-container {
-moz-appearance: none !important;
min-width: 180px !important;
max-width: 180px !important;}

/* Remove wasted space from tab buttons */
.tabbrowser-tab {
border: 1px solid #000 !important;
max-width: 350px !important;
margin: 0px 0px 0px 0px !important;}

.tab-close-button {
margin: 0px 0px 0px 0px !important;
padding: 0px 0px 0px 0px !important;
border: 0px !important;
margin-right: 0px !important;
border: none !important;}

.tab-throbber, .tab-icon {
margin: 0px 0px 0px 0px !important;
padding: 0px 0px 0px 0px !important;
border: 0px !important;
margin-left: 0px !important;
-moz-margin-end: 0px !important;
border: none !important;}

.tabbrowser-tab[fadein]:not([pinned]) {
  min-width: 16px !important;
  max-width: 320px !important;
}
#alltabs-button {visibility: visible !important;}

/*Dialog Boxes*/
/*Size of Options/Preferences window*/
#BrowserPreferences {width: 1000px !important; height: 600px !important;}

/*Unlist Site Settings*/
listitem[preference="privacy.cpd.siteSettings"] { display:none!important; }
#SanitizeDialog checkbox[label="Site Preferences"] { visibility: hidden !important; }

/*Global UI font*/
 * {font-family: Trebuchet MS !important;
font-size: 15px !important;
}

.statuspanel-label {min-width:700px !important;}

#urlbar[level="broken"] #identity-box {background-color: red !important;}

#addon-bar[mode="full"] .toolbarbutton-text {display:none !important;}

/* Remove the Bookmark star */
#star-button {display: none !important;}

/*Remove Go button*/
#go-button {display: none !important;}

/*.tabs-newtab-button {display: none;}*/

#addonbar-closebutton {
    visibility: collapse !important;}

That is my userChrome.css file listing.

Nothing jumps out at me further up the file as being likely to countermand that rule.

Note that you start the file with "/ *" and you should remove the space to get a valid comment: "/*"

/ * Do not remove the @namespace line -- it's required for correct functioning */

Good catch. I have removed the extra space. Since that line is not valid code, I would expect that it was ignored anyway.