Is there a shortcut for switching between 2 open tabs back & forth?

For Firefox the add-on Tab Utilities Lite can be used to get the required behaviour:

Ctrl-Tab to navigate tabs in most recently used order, Ctrl-Left/Right Arrow in sequencial order

Ctrl+Shift+Tab: to reverse cycle order.

And for the nine first tabs you could use Ctrl+1, Ctrl+2, etc.

It works in Firefox, Google Chrome and Internet Explorer.

Here is a list of hotkey whe working with tabs in IE8:

The following table describes the keyboard shortcuts that are available when working with tabs.

• Open links in a new tab in the background: CTRL while clicking the link.
• Open links in a new tab in the foreground: CTRL+SHIFT while clicking the link.
• Open a new tab in the foreground: CTRL+T or double-click an empty space on the tab row.
• Open a copy the current tab in a new tab in Internet Explorer 8: CTRL+K
• Switch between tabs: CTRL+TAB to move forward or CTRL+SHIFT+TAB to move backward and CTRL+1-2-3-4-5 to switch from one to another
• Close the current tab (or the current window when there are no open tabs): CTRL+W or ALT+F4
• Open a new tab in the foreground from the Address bar: ALT+ENTER
• Switch to a specific tab number: CTRL+n (where n is a number between 1 and 8)
• Switch to the last tab: CTRL+9
• Close all tabs except for the one you're viewing: CTRL+ALT+F4
• Open Quick Tabs (thumbnail view): CTRL+Q

Source

In Opera, no change or shortcut is needed. In all versions of Opera Ctrl + TAB switches between two tabs. The tabs don't even have to be adjacent; it is the last two shown. This works much like ALT + TAB to switch between Windows applications.

(The shortcuts for cycling through the tabs in Opera were 1 and 2 in older versions. It is a little more complicated by default in newer versions, but I am using these old shortcuts.)