violentmonkey | Violentmonkey provides userscripts support for browsers | Browser Plugin library

Judging by the source code in other extensions like Stylus or Violentmonkey (beta) that recently added the same feature:

1. The background script makes a Blob in the background script with the data, gets its URL via URL.createObjectURL, puts it into a Set-Cookie header via chrome.webRequest API.
2. The content script reads the URL from document.cookie and uses it in a synchronous XMLHttpRequest to get the original data synchronously.

This trick is based on an answer by Xan.

You can read the .innerText property of the body, then use String#includes to see if your phrase is present.

If it is present you can return out of the function to end the script.

Something like this:

The problem here seems to be that the rating graph (incl. the average rating) is loaded asynchronous and thus is not present when your script runs.

In order to counter this, you could listen for changes of the parent .sidebar element and then remove it as soon as it's present:

Maybe so...

The second method below usually works but something seems to be interfering with it on your page for some reason. An ugly workaround is to put an empty hljs property on the window in advance, so that the page script, when run, thinks it already exists and does nothing:

To start with, remove the glob as it can cause issues.

Web Console is part of browser scope while a GreaseMonkey script is injected into a page content and its scope it limited to that page only.

You can open a new tab/window from page content with GM but that window/tab will have its own page content scope which is separate from the first one. Therefore, you can not access properties belonging to another tab/window from a page content of different tab.

This separation is for all content scripts and not only for GM. Otherwise, it would have created a great security risk if JS in one tab/page could access data on other tabs.

Update:

As pointed out by wOxxOm, the CSP and cross-origin policy prevents Cross-Origin Resource Sharing (CORS).

In your example, in Firefox, the first issue is that Firefox will "Block pop-up windows" from the content script.

Testing Firefox dedicated userScripts API on FireMonkey, after allowing popup, still causes an error:

SecurityError: Permission denied to access property "onload" on cross-origin object

In this case, I believe it is due to the sandboxing of userScripts for security.

GM/TM/VM do not use the userScripts API and use other means to inject userSripts which results in different outcome.

storage.sync

Represents the sync storage area. Items in sync storage are synced by the browser, and are available across all instances of that browser that the user is logged into (e.g. via Firefox sync, or a Google account), across different devices.

There are limitations with sync.

• Sync requires login
• Up to 100kb can be synced and more than that fails
• Storage sync usually syncs the entire extension storage and in case of user-script managers, they are often larger than 100kb as they include the user-scripts as well
• Data is stored elsewhere (not on your computer) which can have security/privacy concerns
• Storage read/write requires internet access and longer times due to remote data storage
• Extension must have the sync option

1) Is there a way to handle that with Firefox Sync ?

Yes but depends on aforementioned. While Firefox sync doesn't appear to have the 100kb limit, syncing the entire Firefox takes longer (How do I set up Sync on my computer?).

2) Would an alternative (Tampermonkey, Violentmonkey, ?) handle that better ?

Greasemonkey: No sync feature
Tampermonkey: Sync to Dropbox, Google Drive, or a WebDAV service
Violentmonkey: Sync to Dropbox, OneDrive, Google Drive, or a WebDAV service
Firemonkey: No sync feature

TM/VM 3rd party storage sync allows storage of more than 100KB. However, using 3rd party storage allows 3rd party tracking which is a privacy consideration. The storage read/write operation is also a lot slower.

3) Where can I find the scripts/database in the new Greasemonkey system ?

Since Firefox 57, extensions can not save files to the HD and the only storage area is the extension storage which is not accessible as before.

4) Could I synchronize them via Google drive ? (There seem to be some hacks to sync a file between machines)

Refer to above

5) Would there be a simple, free alternative that would allow me to synchronize a very small file between machines ?

That depends on the set-up and how often data is changed.

For example, if data is not often changed, especially if the data privacy is imperative, there is the backup export/import option (script + data). Therefore, the extension data can be saved to a USB flash drive and loaded anywhere (suitable for shared environment).