Override XBL content attributes with CSS

[HELBLING Andreas - TDHS]

I am trying to change the "flexiness" of the toolbar button labels in Thunderbird.
The toolbar buttons are bound to the following XBL:
(snipped from chrome://global/content/bindings/toolbarbutton.xml)

<binding id="toolbarbutton" display="xul:button" extends="chrome://global/content/bindings/button.xml#button-base">
<resources>
<stylesheet src="chrome://global/skin/toolbarbutton.css"/>
</resources>
<content>
<children includes="observes|template|menupopup|tooltip"/>
<xul:image class="toolbarbutton-icon" xbl:inherits="validate,src=image,toolbarmode,buttonstyle"/>
<xul:label class="toolbarbutton-text" crop="right" flex="1" xbl:inherits="value=label,accesskey,crop,toolbarmode,buttonstyle"/>
</content>
</binding>

I can see that a toolbar button widget has a label with flex="1", which is exactly what I want to override. So far, no luck.
In an overlay to messenger.xul the following CSS is included:

.toolbarbutton-1 > .toolbarbutton-text
{
-moz-box-flex: 0;
}

-The DOM Inspector shows this style for the anonymous xul:label element (-moz-box-flex = "0")
-There are no other CSS style definitions for this element that define the -moz-box-flex style
-The computed CSS style for -moz-box-flex is also "0"
-The "flex" attribute on the xul:label DOM node is "1"
-The label of the toolbar buttons remain flexed
-No errors in the error console

Any ideas?
Thanx


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