Q:

See when tab was last refreshed in Safari

Let's say I open a website. I then later open another tab that goes to the same address.  
How can I see when each tab was last refreshed?

A:

If you enable the Develop menu, you can view the HTTP headers when you press Cmd-Opt-I. You need to always enable resource tracking.

It's the Last-Modified value.
This is the same date as shown in Firefox's info window (even though it's not the actual load time, but you were happy with it before).

The following AppleScript reads data from Safari's History file and shows a popup with the date recorded there.

tell application "Safari"
    set u to URL of current tab of (first window whose index is 1)

    set t to do shell script "/usr/libexec/PlistBuddy -c Print ~/Library/Safari/History.plist | grep -A 50 '= " & u & "' | grep lastVisitedDate | head -n1 | cut -d= -f2"

    if t = "" then
        display alert "Could not find " & u & " in history!"
        return
    end if

    set s to "date -r $( echo '946771200 + " & t & "' | bc | cut -d. -f1 )"
    set d to do shell script s
    display alert d
end tell

You can use it as part of a Service you create in Automator (no input, in Safari, using a single Run AppleScript action), or as regular AppleScript from the Scripts menu you can enable in AppleScript Editor. In the latter case, place the scpt file in ~/Library/Scripts/Applications/Safari.

Some caveats:

It uses the time Safari records for its history. Simply reloading a page does not update the Safari history record for the URL.
It does not work with URLs only opened in Private Browsing, as they're not recorded (duh)
I'm not sure about the offset 946771200 (which is January 1 2000, at 6 pm). If the dates of your recently viewed pages are off by a few hours, add or subtract the number of seconds in those hours from that value.

