firefox: add keys to modify in about:config

firefox/README.md

@@ -3,6 +3,13 @@ Firefox configuration
 
 - Configure Firefox Sync if needed
 - Change to dark theme and compact density in Customize and Tree Style Tab config
+- Access about:config and modify the following keys:
+```
+# https://support.mozilla.org/en-US/questions/1278533
+ui.key.menuAccessKeyFocuses = false
+# https://www.userchrome.org/how-create-userchrome-css.html
+toolkit.legacyUserProfileCustomizations.stylesheets = true
+```
 
 Add-ons
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