FireFox change their settings with every update. I cant actually find what i've read (yeah, i know) but some newer versions of Firefox only retain history for 90 days. Keeping all this data takes up memory and as you can see 9 months (40 weeks) of history is going to be alot more data then just bookmarking relevant sites.
You are still vague with your posts. You say "I know that if its not showing up, I delete my browsing history".
If what is not showing up. The site you are looking for OR the whole history.
If it only remembers 90 days and what you are after was before then, it will be gone.
If a site is important to you, bookmark it and back up your bookmarks.