Hmm. I can replicate this. This looks like a regression of bug 1823791 that sets a default width of 125px to the search bar.

• 1823791 - Flexible Spaces are taking more space in the toolbar than they used to

• https://hg.mozilla.org/mozilla-central/rev/e08af0bae48c

 /* Search bar */
 
 #search-container {
   min-width: 125px;
+  width: 125px;
 }
 
 #search-container,
 #wrapper-search-container {
-  flex: 175 175;
+  flex: 175 175 auto;
 }

As a fix for now you can disable this width rule via userChrome.css

#widget-overflow-mainView #search-container { width:unset !important; } /*113*/

EDIT: I've modified the code to make it work only for the overflow area, just to be sure.

It is not that difficult to create userChrome.css if you have never used it.

The first step is to open the "Help -> Troubleshooting Information" page and find the button to access the profile folder with the random name (xxxxxxxx.default-release).

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

You can find the button to go to the profile folder under the "Application Basics" section as "Profile Folder -> Open Folder". If you click this button then you open the profile folder in the Windows File Explorer. You need to create a folder with the name chrome in the profile folder with the random name (name is all lowercase). In the chrome folder you need to create a plain text file with the name userChrome.css (name is case sensitive). In this userChrome.css text file you paste the text posted. On Mac you can use the TextEdit utility to create the userChrome.css file as a plain text file.

In Windows saving the file is usually the only time things get more complicated because Windows can silently add a .txt file extension and you end up with a file named userChrome.css.txt. To avoid this you need to make sure to select "All files" in the dialog to save the file in the text editor using "Save File as".

You need to close (Quit/Exit) and restart Firefox when you create or modify the userChrome.css file.

More info about userChrome.css/userContent.css in case you are not familiar: *https://www.userchrome.org/what-is-userchrome-css.html

• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets = true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Thank you, cor-el!

I did as your instructions, and the result was as expected: the width is now normal, full size. ✔

Note that this has been fixed for 115 via bug 1833318.

• 1833318 - Search box made shorter after update to FF 113.0
• https://hg.mozilla.org/mozilla-central/rev/0d0ce4fe35dd

With this code for userChrome.css to set width:auto in a panel.

#search-container[cui-areatype="panel"] {
  width: auto !important;
}