Sticky: How to fix many problems -

[Devon J]

Standard Troubleshooting procedure:

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In some cases, your download history file can become corrupt. In this case you need to close firefox. Open your firefox profile folder.

Delete downloads.sqlite. Restart firefox. (your downloads.sqlite file will be recreated)

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Make sure your download retention settings are correct

Go to the special Firefox URL about:config
Type retention into the Filter: field,
Make sure browser.download.manager.retention is set to 2.
You can edit the value by double-clicking on the row.
(This MUST be 2 for Download Statusbar to work correctly)

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Another thing to check -

Are there errors in the error console?

First turn on error reporting
Go to the special Firefox URL about:config
Type showInConsole into the Filter: field,
Make sure javascript.options.showInConsole is set to true.
(You can toggle the setting by double-clicking on the row.)

Now close and restart Firefox.
Open the Error Console in the Tools menu.
Look for suspicious errors that originate from a "downbar" file.
Copy and paste the error -

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If nothing else works,
starting a fresh Firefox profile should fix it
See here for information on creating a new profile and moving your old data:
support.mozilla.com/en-US/kb/Recovering+important+data+from+an+old+profile