Does your system have an UltraHD display? The Flash plugin does not scale correctly with higher-than-HD displays. This is a longstanding issue... for example, from about two years ago: [https://support.mozilla.org/questions/982635 Playback controls on YouTube and other video players are tiny] (Yoga Pro 2 at 3200 x 1800). Since Flash is something of a legacy product now, I don't think that Adobe is going to fix this, although, you never know... For another thread last year, I discovered possible workarounds for other Adobe software, but I didn't find anyone applying this idea to the Flash player plugin or, if appropriate, to Firefox's plugin-container.exe process, so I don't hold out a lot of hope: * https://forums.adobe.com/message/6042335#6042335 (post #76) * http://www.danantonielli.com/adobe-app-scaling-on-high-dpi-displays-fix/ Another recent thread discussing that possibility: [https://support.mozilla.org/questions/1101233 Windows 10 scaling problem]