If you use Firefox to edit a post, it may happen in some cases, especially on long posts and when switching between tabs, that the text gets truncated to an unspecified length. If you save this post, the cut content is lost forever.
Check your post before you save it
This behavior happens from time to time, so when you edit a post scroll down to the end of the textarea and check that it is complete every time before you hit the "Save" button!
If it’s not complete, copy the edited text and paste it into Notepad. Than reload the page until the textarea shows the complete post and merge it with your edited text from Notepad.
More about this bug
You can keep track of this problem with the bug report on bugzilla. Introductions to test or confirm this behavior are available on this comment, which clearly shows that this is not a WordPress fault.
At the moment there is no solution but a workaround: You can install the IE Tab extension for Firefox and add "http://www.yourblog.com/wp-admin/post.php" to the list of sites which should be opened in Internet Explorer.