I see the notification "Warning: The tool may already be active in another browser window or tab"
The tool can be only active in one tab at a time. The underlying data structures will get scrambled when they are operated on from two tabs simultaneously. It does not matter whether one or more browser windows are used. Even when you only use a second tab for viewing without editing, permanent damage to your project data can result.
To view or edit two projects simultaneously, use a second web browser. For example, use Chrome in addition to Firefox. Additional advantage: it is even possible to operate on the same project in both browsers.
It is possible that you see this warning without cause. For example when a browser has crashed without shutting down properly. In this case check that you really do not have other browser windows or tabs using the Raster tool. Then, you can ignore the warning.