Proof of failure of a PouchDB live changes feed in a WebExtension. Try open this in two tabs (click the browser action button twice), add some items in one, and check if the other receives the changes.

background.js

chrome.browserAction.onClicked.addListener(() => {
    chrome.tabs.create({url: '/testchanges.html'})
})

manifest.json

{
    "manifest_version": 2,
    "name": "test pouchdb changes",
    "version": "1.0",
    "description": "Different tabs/processes do not receive live changes when the extension requests 'storage' permission. Try open this in two tabs (click the browser action button twice), add some items in one, and check if the other receives the changes. Fails at least in Firefox 50 and Chromium 55, with PouchDB 6.1.0.",
    "browser_action": {},
    "background": {
        "scripts": ["/background.js"]
    },
    "permissions": [
        "storage"
    ]
}

test.js

const db = new PouchDB('testdb')

db.changes({
    live: true,
    include_docs: true,
}).on('change', change => {
    let text = "\n" + JSON.stringify(change.doc)
    document.getElementById('changes').innerText += text
}).catch(e => {
    console.error(e)
})

function addItem(letter) {
    db.put({
        _id: new Date().toJSON(),
        letter: letter,
    })
}

document.getElementById('A').onclick = () => addItem('A')
document.getElementById('B').onclick = () => addItem('B')

testchanges.html

<button id="A">add A</button>
<button id="B">add B</button>
<h3>Changes:</h3>
<pre id="changes"></pre>

<script src="/pouchdb.min.js"></script>
<script src="test.js"></script>