Bug report: the window with the avast! Online Security options is not resizeable and can't be maximized. So, I can't fully customize the add-on in Firefox. I simply don't see all the options.
It seems that I found the cause of the bug (the screenshot is re-attached to this message).
I always refuse to install Google Chrome and as usual I unchecked the browser installation offers in Avast setup window. But when I started my Firefox, I detected the above described bug with avast! Online Security options. Other users told me that the options should be opened in a tab, not a separate window, but I couldn't manage to do that. The online service isn't vital for me, so I repaired my Avast and removed it.
A bit later I started my Speedyfox tool to optimize my Firefox and Thunderbird. I was surprised to see in the tool's window that I have a Chrome profile! Namely it was a file at the following address:
C:\Users\<current_user>\AppData\Local\Google\Chrome\User Data\Default\extensions\WebNavigation.crx I checked my computer but didn't find any traces of Chrome except for that file in AppData folder. I manually removed the Chrome folder with its content and Speedyfox now detects no more Chrome profiles.
I didn't try to reinstall Online Security (a useless feature in comparison with Ghostery, BitDefender Traffic Lights and WOT) to check everything but I believe that the trace of Chrome in AppData folder was the cause of the bug. Something is still wrong with Avast installer.