How to remove the Close Multiple Tabs and Select All Tabs commands from Firefox's tab context menu - l10nelw/select-tabs GitHub Wiki

Reasons to do it

You never use them.
Close Multiple Tabs becomes redundant now that you can Select Tabs > To the Left/Right and then Ctrl+W.
So that Select Tabs doesn't have to share the S access key with Select All Tabs.

If you already have a working userChrome.css setup, skip this section.

Enter about:config in the address bar to access advanced preferences (you may be asked an "accept the risk" confirmation).
Search for the toolkit.legacyUserProfileCustomizations.stylesheets setting, which enables support for userChrome.css.
If the setting is not true, double-click it to toggle it.
Enter about:support in the address bar to find your Profile Folder. Open it.
In your profile folder, create a folder named chrome, and inside it create a plain text file named userChrome.css.

Add the following rule:

/* Remove items from tab context menu */
#context_closeTabOptions,
#context_selectAllTabs
{ display: none !important; }