I was able to reproduce this issue on Ubuntu 24.10 using Fx 135.0.1, Fx 136.0b9, and the latest Nightly 137.0a1. Make sure that the bookmark toolbar is full and "Show more bookmarks" is displayed, drag a new URL, and drop to the "Show more bookmarks". The window opens to the right side of the screen before dropping the URL.
Bug 1949992 Comment 3 Edit History
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I was able to reproduce this issue on Ubuntu 24.10 using Fx 135.0.1, Fx 136.0b9, and the latest Nightly 137.0a1. Make sure that the bookmark toolbar is full and "Show more bookmarks" is displayed, drag a new URL, and drop it to the "Show more bookmarks". The window opens to the right side of the screen before dropping the URL.