addonsfirefox
Is there a possibility to hide all borders and bars in firefox like address bar, navigation bar, window title and tabs, so the remaining area is just the content area of the browser?
I would like to make a movie of a web application I wrote, but have to film it in a bigger resolution than the app itself. I simply want to show the app on a background without all the browser stuff.
The best method would be via keyboard shortcut, so that I can reenable all the stuff again.
Hit the Alt key. This will open the normal Firefox menu.
I would suggest avoiding the issue altogether by preventing video/music from automatically playing. Here are three things you can do (together or separately) to help prevent auto playing video/sound.
Install AdBlock Plus, which will block most advertisements including those that automatically start playing video/sound.
Go to about:config and set media.autoplay.enabled to false. This supposedly stops HTML5 media from autoplaying, though I couldn't find any references to it in the about:config entries on MozillaZine Knowledge Base.
Go to about:config and set plugins.click_to_play to true. As you can see, it stopped Youtube from auto-playing this annoying video. 
Best Answer
The easiest way is to install relevant add-ons in Firefox.
For any of the following setup, install the respective add-ons. Firefox may need to restart, but often unnecessary for Firefox Quantum. The shortcut key, if available, can be changed from Add-ons Manager > Extensions for respective add-on.
Firefox has a redesigned keyboard shortcuts section to manage all shortcuts in one place, which has been made available since Firefox 66.
For Firefox Legacy
Setup A uses the following add-ons:
For example, I have used separate keys F3 and F4 to toggle the navigation menu and tab bar (via Hide Navigation Bar and Hide Tabbar) respectively. To hide the bars, press the assigned keys one by one. This will give Firefox with visible title bar or window decoration only.
Limitation: Assigning the same key does not work well, because of some delay in between. The result is similar even if the separate keys were pressed at the same time. Either way, one of the bars will not hide and play hide-and-seek with the user.
Works with Firefox 56 and earlier (Deprecated), but no longer available from Firefox Add-ons. Tested altogether with Firefox 41.0 on Linux. No screenshot.
For Firefox Quantum
Setup B uses the following add-ons:
Go to a web page on HTTP, HTTPS, or localhost, then use the keyboard shortcut or click the corresponding icon on toolbar or "New window without toolbar" in context menu. The web page will additionally open in a new window with visible title bar and vertical scroll bar only.
Keyboard shortcut: Shift+Alt+N (default in 1.3.0)
Limitation: This add-on does not open any page on local filesystem (file:///) and built-in pages (about:*) at all.
Works with Firefox 42 and later. Tested 1.3.0 with Firefox 69.0 on Linux.
Setup C uses the following add-ons:
Go to a web page, then click the corresponding icon on toolbar or either "Move current tab to pop-up window" or "popup/merge current page" in context menu. The web page will move to a new window with visible title bar and vertical scroll bar only. To revert the new window to a normal tab, click again on "popup/merge current page" in the context menu.
Keyboard shortcut: Not assigned (default in 0.0.8)
Limitation: The pop-up window will always stay on top of original window of Firefox, which can not be changed.
Works with Firefox 53 and later. Tested 0.0.8 with Firefox 69.0 on Linux.
For Firefox without add-on
I have posted a separate answer at below (click or scroll down) that works for any release of Firefox, but not the easiest way and has no keyboard shortcuts.