Safari cannot be forced to quit

I am trying to force to quit Safari but that doesn't work.

What is happening?

When looking at the Dock, the icon Safari is not accompany by a dot (meaning that the application is closed). However, on the Force Quit Application window Safari is visible. I can select it and press Force Quit but nothing happen. I can also make a secondary click on the icon on the Dock and click on Force Quit but again nothing happens. Under Activity Monitor there is no process that contain the word Safari and there is no process that are particularly demanding neither in terms of CPU nor in terms of RAM. When I try to restart my computer, I get a message saying that the computer cannot restart because I have to quit Safari first. When I try to open Safari (via the Dock or via the Applications folder), nothing happens.

How did this situation come to existence?

I already had a few troubles with Safari in the past months. New tabs and windows sometimes load forever and Safari is sometimes very slow. I went camping for a few days and I let my computer under sleep with the Safari, Sublime Text 2 and Preview open. I just came back from camping, I opened my computer and Safari was very very slow. I wanted to quit, I had to force to quit. I thought it worked out well at first place and I then realized that Safari is not "completely closed".

Extra info

My (almost 5 years old) MacBook Pro runs on OS X Yosemite 10.10.3

After @slick's answer the version of Safari:

$cat /Applications/Safari.app/Contents/version.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>BuildVersion</key>
    <string>1</string>
    <key>CFBundleShortVersionString</key>
    <string>8.0.5</string>
    <key>CFBundleVersion</key>
    <string>10600.5.17</string>
    <key>ProjectName</key>
    <string>WebBrowser</string>
    <key>SourceVersion</key>
    <string>7600005017000000</string>
</dict>
</plist>

The only plug-in I have is AdBlock.

Question

What is happening? How can I solve this issue?

I've had this problem in the past, and what works for me is typing into the terminal, "pkill -f safari". That will kill the application no matter what.