In Firefox I can create a fullpage screenshot when I go to the web developer toolbar (can be activated in the developer tool settings). Unfortunately, I can do this only with the default resolution. Is there a chance to get images of higher resolution anyhow? With Firefox or an add-on? Or is it possible in another browser?
In Firefox 62 and following, the GCLI was removed and the screenshot command was moved to the Web Console and prefixed with a colon.
To take a screen shot at 4 times the native screen resolution, open the web console (☰ -> More tools -> Web Developer Tools -> Console or CtrlShiftK, CmdOptionK on Mac or F12 -> Console tab) and type:
:screenshot --dpr 4
The file name is now optional and the console will tell you where the file was saved (typically in 'Downloads' with a system-specific naming scheme).
Additional parameters are available, see the Web Console documentation and Erik Meyer's blog post for more.
An additional note: large DPR values don't always work. There seems to be a limit around 120 megapixels. Larger screenshots will simply not complete and do nothing, silently. If the command does nothing for you, try lowering the dpr or resizing the window, until you get something out.