I'm using the newest version of FVD Speed Dial on Firefox 53.0.3 and spotted this ugly bug: when I use private mode, FVD shows up, but other than having my dials, it behaves like it only launched for the first time (I get the welcome screen) and all the settings are restored to defaults.
The settings also mostly don't work in private mode - I can't change tabs and clicking some of the buttons has no effect whatsoever.
The worst thing is, after launching FVD in private mode, the background is reset to the default in standard mode too.
I hope you can work out these kinks.
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