The icon for Zotero has moved from the bottom of your screen to the upper right of the Menu bar (you can also use the <control><shift><z> keyboard shortcut). As of Zotero 4.0.27 it looks a little different now, as Zotero has "combined the address bar icon and the “Create Web Page Item from Current Page” button into a single save button" as shown below:
If you have a lot of Firefox add-ons and extensions, or are using a smaller monitor than usual, your Zotero icon may have been pushed off your viewable browser screen as shown here:
Note: See box below for info on this topic if you are using a version of Firefox that is 28 or lower (although you shouldn't be...;-)
Second Note: If you have just installed Zotero (in post-29 Firefox) and you don't see the icon, just close and re-open Firefox and you should see it.
What about Chrome? If you have installed Zotero Standalone and the Chrome Connector but your Zotero icon is not appearing when it should, make sure you are running the latest version of Chrome, then delete the Zotero connector, re-install it, then re-start your computer. That usually fixes it - if it doesn't, post on the Zotero forums for assistance.
FOR FIREFOX VERSIONS 28 AND BELOW ONLY (if you've upgraded to Firefox 29, see box above):
It happened to me too ;-) What you need to do is:
1. Confirm you are using the latest version of Firefox (I thought mine updated automatically, but it turned out for some reason it didn’t and it was way out of date)
2. Download the newest version of Zotero if you do not have it already (Zotero should automatically update, but sometimes it doesn't)
3. Check the Help page from Zotero about this issue: http://www.zotero.org/support/kb/no_status_bar_icon
Don't forget that in a pinch you can use the Control-Forward Slash keyboard shortcut to open or close the little frame at the bottom of your screen that shows the Zotero icon: hold down the <control> key and press the forward slash (/) key.
Even if you don't see the Zotero icon at the bottom of your browser screen, Zotero is always there and working in the background (you don't even need an Internet connection - a long plane ride is a great opportunity to clean up your Zotero records :-)