/* Reduce left spacer on non-maximized windows */
.titlebar-spacer[type="pre-tabs"] {
  display: none !important;
}

Hi Muhammad Hussein Ammari, the usual method is to apply a custom style rule in a userChrome.css file. It's a little bit of a project, so make sure you set aside 10 quiet minutes to work through it. (For anyone who already has a userChrome.css file set up, you just need to add the rule under (A) to your file.) '''(A) Select and copy the following style rule code''' <pre>/* Reduce left spacer on non-maximized windows */ .titlebar-spacer[type="pre-tabs"] { display: none !important; } </pre> '''(B) Generate and download a''' <code>userChrome.css</code> '''file''' Open the following page and paste the above rules into the editor, replacing the sample rule: https://www.userchrome.org/download-userchrome-css.html Then click "Generate CSS File" and save the userChrome.css file to your computer. (See first attached screenshot) Use the downloads list on the toolbar to open the downloads folder directly to the new userChrome.css file. (See second attached screenshot) Minimize that file browser window for later reference. '''(C) Create a new''' <code>chrome</code> '''folder in your profile folder''' The following article has the detailed steps for that (#1, #2, and I recommend #3) https://www.userchrome.org/how-create-userchrome-css.html ''I have videos for both Windows and Mac in case the text is not clear.'' '''(D) Move the''' <code>userChrome.css</code> '''file you generated in Step B into the''' <code>chrome</code> '''folder you created in Step C''' '''(E) Set Firefox to look for userChrome.css at startup''' -- see step #6 in the above article. The next time you exit Firefox and start it up again, it should discover that file and apply the rules. Success?

Ok, Thanks