How did you "customize" the search bar?
If is was via the optional userChrome.css file, open that file and edit the number that specifies the custom length of of that 'bar'.

https://www.userchrome.org/what-is-userchrome-css.html

See this to learn where the userChrome.css file is located in your Profile.
https://www.userchrome.org/how-create-userchrome-css.html
https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Hi, if you were resizing by dragging the space between the URL and search bars, it would seem that this is a bug introduced in v76, but fixed in v77 - only a month to wait for v77!

https://old.reddit.com/r/firefox/comments/ge14ag/version_760_resized_addresssearch_bar_no_longer/

Are there items between the location bar and the search bar ?

Try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I am hoping with the next update this problem will be solved. I use the search bar a great deal and the results open in a new TAB. This is good when researching an item, so as to compare each TAB result. It would be good to be able to resize the search bar in the customisation facility and not to "default".

I agree with you, Gary. Let's hope the functionality of resizing the address and search bars is restored in the next update.

I'm not having a problem with resizing the location bar and search bar in Firefox 76 during the session and after restarting Firefox.

I see this data stored in xulstore.json under the "main-window" key, so you can check this file with Firefox closed. Min width for search bar:

• "urlbar-container":{"width":"774"}
• "search-container":{"width":"125"}

Max width for search bar:

• "urlbar-container":{"width":"386"}
• "search-container":{"width":"484"}

This may not work with features like Private Browsing mode or "Resist Fingerprinting" (haven't tested this).

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Previously, we could just drag each box with the mouse to make it shorter or longer. Hope that function is restored in the next updated version of Firefox.