Drag-and-drop into new window fails if user has custom home page without scheme

[dsuedholt]

Steps to reproduce

• Open Firefox in new profile
• Install Sidebery
• Set home to any domain without a scheme, e.g. "mozilla.org" or "google.com", NOT "https://mozilla.org" or "https://google.com"
• Drag-and-drop a tab outside of the sidebar

Expected behavior

• tab opens in new window

Actual behavior

• nothing happens
• check the browser toolbox to see an exception:

Error message

[Exception... "Component returned failure code: 0x804b000a (NS_ERROR_MALFORMED_URI) [nsIIOService.newURI]"  nsresult: "0x804b000a (NS_ERROR_MALFORMED_URI)"  location: "JS frame :: chrome://browser/content/parent/ext-windows.js :: setContentTriggeringPrincipal :: line 254"  data: no] [ext-windows.js:254:27](chrome://browser/content/parent/ext-windows.js)
    setContentTriggeringPrincipal chrome://browser/content/parent/ext-windows.js:254
    create chrome://browser/content/parent/ext-windows.js:353
    result resource://gre/modules/ExtensionParent.sys.mjs:1135
    withCallContextData resource://gre/modules/ExtensionParent.sys.mjs:611
    result resource://gre/modules/ExtensionParent.sys.mjs:1134
    withPendingBrowser resource://gre/modules/ExtensionParent.sys.mjs:621
    result resource://gre/modules/ExtensionParent.sys.mjs:1133
    callAndLog resource://gre/modules/ExtensionParent.sys.mjs:1084
    recvAPICall resource://gre/modules/ExtensionParent.sys.mjs:1132
    AsyncFunctionNext self-hosted:852
    (Async: async)
    _recv resource://gre/modules/ConduitsChild.sys.mjs:77
    receiveMessage resource://gre/modules/ConduitsParent.sys.mjs:469

This is related to unexpected behavior in the Web Extension API when calling chrome.windows.create with both conf.url and conf.tabId set to null: https://bugzilla.mozilla.org/show_bug.cgi?id=1882822

This can be worked around by setting conf.url to pretty much any valid URI, e.g. 'about:blank'.