How to change 'screenw' ?

Because of my double screen desktop on Ubuntu, my flash player resolution on youtube is 2960px. ( one screen with 1680px another with 1280px.)

This is the debugging information :

cl=77351737&ts=1412872329&sourceid=y&plid=AAUFLI3ogFwma62z&w=854&ptk=EthosLab%252Buser&droppedFrames=75&idpj=%2D1&hasstoryboard=1&c=WEB&next%5Fid=DG81819BTZs&sdetail=p%253A%252Ffeed%252Fsubscriptions&el=detailpage&tpmt=369&cr=FR&nsivbbl=718311&vw=854&of=MC98C44MS1A%2DCUk6Labcfw&cfps=31%2E007751937984494&h=480&cbr=Firefox&nsiabbl=119567&screenw=2960&hl=fr%5FFR&vh=480&rendering=software&screenh=1050&lact=6014&debug%5FflashVersion=LNX%2011%2C2%2C202%2C406&decoding=software&volume=0&debug%5FvideoId=DG81819BTZs&debug%5Fdate=Sat%20Oct%2011%2023%3A49%3A50%20GMT%2B0200%202014&ei=eKQ5VOCwN8ey0QW6v4DQCQ&tspne=0&debug%5FsourceData=&fmt=135&tspfdt=977&cbrver=31%2E0&referrer=https%253A%252F%252Fwww%2Eyoutube%2Ecom%252Ffeed%252Fsubscriptions&afmt=140&scoville=1&tsphab=1&vq=auto&nsidf=75&playerw=854&debug%5FplaybackQuality=large&mos=1&pixel%5Fratio=1&pd=0%2E5409999999999937&cver=as3&vis=0&videoFps=31&md=1&playerh=512&subscribed=1&csipt=watch%2Cwatch7ad&stageFps=10&ldpj=%2D39&fexp=900225%2C907257%2C908584%2C909708%2C910824%2C914070%2C916941%2C927622%2C930666%2C931983%2C931987%2C932404%2C934030%2C934804%2C946023%2C947209%2C952302%2C952901%2C953801&cos=X11&cpn=z7Y%5FgEU%5FtIkdchT4

Does anyone have an idea to fix this problem ?

then00b