Hmm can't edit. Anyway I've been using Firefox for about 16 years now and only in the past three years has there been undesirable UI changes forced in updates.

In Firefox 91+ you can no longer disable Proton via "browser.proton.enabled = false" on the about:config page. Best is to leave browser.proton.enabled at its default true value as the false setting can cause issues.

In Firefox 91+ you will have to use code in userChrome.css to override changes coming with the Proton design.

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/firefox-89-styling-proton-ui.html

Not a solution

I've got things working as normal again.

Basically Check a setting in Firefox about:config Access profile,create a folder, create a file and you do not need to write code because I have supplied the text, you just copy and paste. So work through instructions step by step. Note: I have added an image below to show process. If you cannot see details, access forum and click on image to enlarge.

• Open 'about:config'

Info at link if unsure but basically type about:config in the address text bar. https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• in search type: legacy
• look for: toolkit.legacyUserProfileCustomizations.stylesheets
• make sure it is set to 'True'

Whilst in about:config I search for 'proton' and found 6 items which are all set to false.

Now access the profile itself:

• Menu app icon > Help > More Troubleshooting Information
• About 11th in list is 'Profile Folder' - click on the 'Open Folder' button

A new window opens showing the contents of your Firefox profile.

• Create a new folder and call it chrome - it must be spelt correctly - all lowercase.
• Open a text editor eg: Notepad

• Copy all the text shown between the lines below and paste it into 'Notepad'.

I advise you double check the content shown in the email you receive is the same as the info posted in this forum question.

menupopup > menuitem, menupopup > menu {
  padding-block: 3px !important;
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

• Exit Firefox now - you need to close the browser; this is important.

The Notepad document:

• File > Save As
• Choose where to save it - it must be saved in the 'chrome' folder you just created.
• Save the Notepad file with this name: userChrome.css

All lowercase except for the 'C' which is uppercase.

• Make sure the 'Save as Type' is 'All files' - not a text document.
• click on 'Save'

In the 'chrome' folder, it's type will be described as 'Cascading Style sheet.

Start Firefox

I've posted a bug report. It may help if you could offer comments in support and explain how this effects you. https://bugzilla.mozilla.org/show_bug.cgi?id=1725401

Thanks for that Toad-Hall. It would be nice for users to have a choice without jumping through these hoops and your efforts are appreciated :-)

It sure would be more helpful for the Mozilla team just simply correct this terrible mess with the double spaced bookmark menu.

There is alot of chat/feedback at the link below. As it is a mozilla forum you would expect developers to be monitoring it, but it may be helpful if people could at least click on the votes to increase the number as it may get the attention of someone. https://mozilla.crowdicity.com/post/719980