There are different kinds of problems with missing inventory that can take place; those that are controllable at the viewer or computer end are generally reversible. Problems caused by the Second Life asset server are not always fixable.
If items that you rezzed out in-world are missing, then refer to this page instead.
If your inventory is still not loading fully, then the issue may be a network or connection problem. General suggestions for problems that may be connection-related are:
Back on Firestorm, if your inventory count still looks lower than it should be or stalls in the middle of fetching items, then:
Start by finding your avatar key while logged into Firestorm or another viewer that displays it - see the figure to the right; the UUID is highlighted in green.
It is a string of letters and numbers that appears in your legacy-style profile, though the exact location will vary because different skin options display it differently. If you use web profiles, you will need to switch into the legacy profile to find it. (In Firestorm, this is in Preferences -> User Interface -> Interface Windows → Use web profiles by default.) You can return to the web profile style afterward. When you find your key, just copy it down somewhere.
Now you will need the latest release version of the SL viewer, so if you don't have it installed yet, download and install it. Then, log into the SL Viewer and get your inventory loaded in that viewer. This might work with any other viewer, as well, but the SL Viewer is the only one we've tested with. Once your inventory is loaded there, log out.
Locate your Second Life cache folder and your Firestorm cache folder on your hard drive. Our clearing cache page will help you locate where that is (just replace “Firestorm” with “SecondLife” in the path name. Don't clear either cache! Just use the paths shown to find the folders.
In the Second Life cache folder, find the file named with your avatar key and ending in “inv.gz”. Drop or copy this file into your Firestorm cache folder. Allow it to replace the one there.
Log into Firestorm and check to see if your inventory is there.
Avoid clearing cache or you may need to do this process again.
Please note: Under normal circumstances, you should not share cache between viewers because it can cause other weird problems. This suggestion should not be read as an indication that setting your cache in different viewers to the same place would be useful. It won't be.
For additional ideas, also refer to the Second Life wiki page on Inventory Loss.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.