Does this also happen if you manually maximize the Firefox window and then click the maximize button >

You can try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.

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

• Help -> More 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

Many users who created a userChrome.css with "tabs below address bar" rules have this problem in Firefox 99. Obviously something has changed position by several pixels. Please return to the source of your rules and check for an updated version, or consider switching to a more updated source for those rules.

You can try the code in this forum response for CSS code in userChrome.css for Firefox 89+ to move the tabs to below the Navigation Toolbar:

• https://support.mozilla.org/en-US/kb/forum-response-tabs-below-url-bar

The forum response isn't maintained so I would update from the source from which you got your code originally. Alternatively, look at the thread linked below (remove blank space in link). https://www.reddit. com/r/FirefoxCSS/comments/u202no/v_99_menu_fix_wanted/

to cor-el see my exemple

• showing FF according with the css I'm working now

• according to the suggested css - the Menu bar is normal now

but the tabs on bottom are partly illegible, and there also is a double line visible now

with below css the Menu bar is normal again, such as the tabs on bottom

1. TabsToolbar {

position: absolute;
display: block;
bottom: 0;
width: 100vw;
background-clip: padding-box;
color: var(--toolbar-color);
} 

1. tabbrowser-tabs {

width: 100vw;
} 

1. navigator-toolbox {

position: relative;
padding-bottom: calc( var(--tab-min-height) + 8px );

}

1. main-window[tabsintitlebar][sizemode="maximized"]:not([inDOMFullscreen="true"]) #titlebar {

height: 36px;
} 

.titlebar-buttonbox-container { position: fixed;

right: 0;
visibility: visible;
display: block;
}
#TabsToolbar .titlebar-buttonbox-container,
#TabsToolbar #window-controls {

display: none;

}