Add note about Firefox extension

Ref #1

README.md

@@ -232,15 +232,15 @@ make install
 
 ## Browser compatibility
 
-| Web browser | Support |             Engine              |
-| ----------- |:-------:|:-------------------------------:|
-| [Chrome]    |    ✓    | [![Blink][chrome.svg]][Blink]   |
-| [Firefox]   |    ✓    | [![Gecko][firefox.svg]][Gecko]  |
-| [Edge]      |    ?    | [![Blink][chrome.svg]][Blink]   |
-| [Opera]     |    ✓    | [![Blink][chrome.svg]][Blink]   |
-| [Safari]    |    ✗    | [![WebKit][safari.svg]][WebKit] |
-| [Vivaldi]   |    ✓    | [![Blink][chrome.svg]][Blink]   |
-| [Brave]     |    ✓    | [![Blink][chrome.svg]][Blink]   |
+| Web browser | Support |             Engine              | Comments |
+| ----------- |:-------:|:-------------------------------:|:--------:|
+| [Chrome]    |    ✓    | [![Blink][chrome.svg]][Blink]   ||
+| [Firefox]   |    ✓    | [![Gecko][firefox.svg]][Gecko]  | [Only developer or unbranded version](https://wiki.mozilla.org/Add-ons/Extension_Signing#FAQ) |
+| [Edge]      |    ?    | [![Blink][chrome.svg]][Blink]   ||
+| [Opera]     |    ✓    | [![Blink][chrome.svg]][Blink]   ||
+| [Safari]    |    ✗    | [![WebKit][safari.svg]][WebKit] ||
+| [Vivaldi]   |    ✓    | [![Blink][chrome.svg]][Blink]   ||
+| [Brave]     |    ✓    | [![Blink][chrome.svg]][Blink]   ||
 
 ## Credits
 

scripts/setup

@@ -192,6 +192,7 @@ print_install_instructions() {
   printf "${cyan}Chrome${none}:\n"
   (cd "$chrome_target"; printf "Open the ${bold}Extensions${none} page by navigating to chrome://extensions, enable ${bold}Developer mode${none} then ${bold}Load unpacked${none} to select the extension directory: %s\n" "$PWD")
   printf "${cyan}Firefox${none}:\n"
+  printf "Only developer or unbranded version (see https://wiki.mozilla.org/Add-ons/Extension_Signing#FAQ)\n"
   printf "Open about:config, change ${bold}xpinstall.signatures.required${none} to ${bold}false${none}\n"
   (cd "$firefox_target"; printf "Open about:addons ❯ ${bold}Extensions${none}, click ${bold}Install add-on from file${none} and select the package file: %s\n" "$PWD/package.zip")
   if test "$INTERACTIVE" = true; then