fix: Improve extraHeaders feature detection (#787)

On Firefox forks the runtime.isFirefox detection was failing because the browser name
was not 'Firefox'. This lead to an error when registering webRequest listeners
as the extraHeaders option is not needed, preventing the extension from loading.

There is an extension API that can be used to detect support for this option: OnBeforeSendHeadersOptions.

This fix updates the feature detection check to explicitly look for this API. 
We also check to for the presence of OnBeforeSendHeadersOptions
as it does not exist on Edge.

add-on/src/lib/ipfs-companion.js

@@ -97,7 +97,7 @@ module.exports = async function init () {
 
   function registerListeners () {
     const onBeforeSendInfoSpec = ['blocking', 'requestHeaders']
-    if (!runtime.isFirefox) {
+    if (browser.webRequest.OnBeforeSendHeadersOptions && 'EXTRA_HEADERS' in browser.webRequest.OnBeforeSendHeadersOptions) {
       // Chrome 72+  requires 'extraHeaders' for access to Referer header (used in cors whitelisting of webui)
       onBeforeSendInfoSpec.push('extraHeaders')
     }