Your user-agent is:
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:45.0) Gecko/20100101 Firefox/45.0
The user agent is a string of text transmitted by a web browser or other application to identify its name and version on the internet. This allows websites and software that uses them to work better together. A website may present one version of itself for user agents that indicate a modern browser, and a simpler version for older ones. User agent strings given by a browser also typically reveal what kind of operating system it is running on.
How do I change my user agent?
The user agent is set client side and can be spoofed (changed) at will, even into something that doesn't make any sense, or just blocked altogether. But beware, a lot of sites use the user-agent string to determine how to best present themselves for your browser. If the websites don't recognize your user agent or the user agent indicates that you're using a very old browser, they may not work and instead just give you a message to upgrade your browser.
With that caveat in mind, changing or blocking your user agent is as simple as going to the plugin/addon store for your browser and searching for "user-agent." You will be given many plugins to choose from with different feature levels. The simplest ones just change your user agent to a string specified by you. Other may feature random rotations, or custom user agents set on a per-site basis, and so on.
Interesting user agents to impersonate once you have a user agent spoofer installed:
This is Google's user agent for indexing the web and using it may result in websites giving you very simple versions of themselves, or even allow you access where normal user agents are kept out with a paywall or forced logins.