Last year, we announced that the Firefox platform team has been working on a new security architecture that separates sites from one another, including separating iframes from different origins into discrete processes. As part of this work, we are deprecating the Canvas drawWindow method because it is synchronous and therefore cannot capture cross-origin iframes with site isolation enabled. We expect to remove the deprecated method sometime in 2022.
We recommend you switch to the tabs.captureTab WebExtension API, which has recently been extended with additional arguments to provide a sufficient replacement for existing use cases.
You can test how your extension behaves with site isolation on the Nightly pre-release channel by going to about:preferences#experimental and enabling Fission (Site Isolation). We plan to gradually enable this feature by default on Firefox Beta in the upcoming months and start rolling it out to a small percentage of release users in Q3.
If you have any questions, please reply to this thread.