#btn_undoclose .toolbarbutton-icon { 
   filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'invert\'>\
   <feColorMatrix in='SourceGraphic' type='matrix' values=\
   '0.38 0.3 0.3 0.0 0.18 \
   0.3 0.38 0.3 0.0 0.0 \
   0.3 0.3 0.38 0.0 0.26 \
   0.0 0.0 0.0 1.0 0.0'/></filter></svg>#invert") !important;
 }

I use a few extensions that add their own toolbar button icons. These icons are multi-coloured and therefore they don't match the built in Firefox icons (which are all a dark indigo colour). I the past, I have manually changed the toolbar icons for these extensions so that they all match. I did this by re-colouring the icons in Photoshop, and then unpacked the xpi file of the extension and replacing the original toolbar button image with my colour-corrected image, This has been working great for me for many years, but it's a annoying to do for extensions that have new versions released regularly (because you have to keep manually replacing the icon after every new update). Now that Firefox is switching to Signed / Validated extensions, this creates a bigger problem,. Manually updating icons inside the extension's xpi file will invalidate the signature, and while it is currently possible to work around this limitation (by setting about:config > using xpinstall.signatures.required to 'false') this option might not be available in future. I therefore wanted to find a different way of dealing with the icon colour issue. After searching on the internet, I found a CSS script that uses "feColorMatrix" settings to colour all the default Firefox icons into various different pastel shades (https://userstyles.org/styles/101231/colorize-firefox-buttons). I used a section of that CSS code and applied it to the icons that were added by my extensions, and it works perfectly! See the attached 'before and after' screenshot image. However, my question relates to how to SPECIFY colours when using feColorMatrix settings. I've investigated online, but it's very complicated and I don't understand it. I was hoping to find a simple online tool where I could just enter an RGB or Hex colour values and have this converted into the required feColorMatrix values. Does such a tool exist? and if not, what values should I use to get icons coloured to match the default built-in Firefox icons? Here is an example of the CCS code that I used (this gives a purple / lilac shade): /* Modify colour of Undo Close Tab icon to - currently set to purple */ #btn_undoclose .toolbarbutton-icon { filter: url("data:image/svg+xml;utf8,<svg xmlns=\'http://www.w3.org/2000/svg\'><filter id=\'invert\'>\ <feColorMatrix in='SourceGraphic' type='matrix' values=\ '0.38 0.3 0.3 0.0 0.18 \ 0.3 0.38 0.3 0.0 0.0 \ 0.3 0.3 0.38 0.0 0.26 \ 0.0 0.0 0.0 1.0 0.0'/></filter></svg>#invert") !important; }