A couple of the less debilitating yet still somewhat annoying downsides I've found:
If you have the "restore previous session" option enabled and have grown accustomed to Firefox remembering all the windows you had open before, you may find it annoying that it no longer remembers the size of your windows; it just puts them to the default size. Although now that I think of it, this might possibly be specific to the X11/Linux version, as other window systems might handle window size in such a way that it's not affected by this.
Also, if you like having websites automatically detect if your system uses a dark color scheme and adjust their CSS accordingly, that no longer works. Again, speaking from an X11/Linux perspective here.
This was infinitely more annoying to me than the endless captchas since due to age and my eyesight getting worse I have different zoom settings for pages I visit.
If you have the "restore previous session" option enabled and have grown accustomed to Firefox remembering all the windows you had open before, you may find it annoying that it no longer remembers the size of your windows; it just puts them to the default size. Although now that I think of it, this might possibly be specific to the X11/Linux version, as other window systems might handle window size in such a way that it's not affected by this.
Also, if you like having websites automatically detect if your system uses a dark color scheme and adjust their CSS accordingly, that no longer works. Again, speaking from an X11/Linux perspective here.