Why does "Check for Updates..." show that there are no updates available while I'm using Firefox 3.6.18 and latest version is 5.0.1?

Question

First and foremost, I'd like to clarify that I hardly have any technical knowledge. So pardon me if this comes across as a silly question to the professionals. Here are the details pertaining to this problem as per the best of my limited knowledge:
I'm currently using Mozilla Firefox 3.6.18; previously it was 3.6.13. It was updated to 3.6.13 from 3.6.9 after I clicked on Check for Updates... and it told me that it'd be done when I reboot my system. When I restarted my PC, I was surprised to find that it was downloading the version 3.6.18. I didn't know how much it'd progressed in the process, so I let it finish it. After that, I disabled the option that enables it to download and update on its own, and turned it off. Next time when I started it, it was 3.6.18, but wasn't updating on its own anymore.
Now I wanna update it directly to the latest version 5.0.1. But when I leave it to "'''Automatically download and install the update'''" option under Tools>Options>Advanced>Update, it downloads and updates the versions as per their release order. So if I leave it to it, then when I start it next time, it'll start downloading the version next to 3.6.18 and update to it instead of updating directly to 5.0.1. It'll keep doing so until it reaches there. For reasons of my own, I don't like that. So I '''unchecked '''the "Automatically download and install the update" option and chose the "'''Ask me what I want to do'''" option (so that it doesn't keep repeating the "downloading and updating" process on its own every time I start my PC). Then I went to Firefox site and thought of downloading and updating to the latest version on my own. But then I was informed that if I do so, '''I'll get the latest version, but I'll lose all my saved bookmarks unless I follow some hectic procedure to save them and load them again after installing the latest version of Firefox'''. I don't want to let that happen either. So, can anyone suggest me a way out of this situation without losing my bookmarks?
Almost forgot. I tried several times by going to '''Help>Check For Updates...''', but every time it returned the same result: "There are no updates available. Firefox will check periodically for updates." How come no update is available while the version I'm using is 3.6.18 and the latest version is 5.0.1? In short, '''Check for Updates...''' didn't help either. (Even when I updated it last time, shouldn't it have updated itself from version 3.6.13 to 5.0.1 directly?)
So, can anyone suggest a way by which I won't lose my bookmarks and would be able to update to the latest version directly too? An answer that wouldn't involve removing the current version and installing the latest one, but directly update to it (it's okay if it needs downloading) would be more welcome. I know that the bookmarks can be saved in a file and all, but I'd like to know if it's possible as it happened for my current version from previous version. (It downloaded and updated itself on its own, disturbing nothing, not even logged me out of certain accounts where I stay logged in. The only problem there was that it wasn't updating directly to the latest version.)
I hope the aforementioned details make sense and are helpful enough for others to help me. Thanks.

Answer 1

Thanks a lot for your swift response. And sorry if it was a bit too hectic to go through my detailed query (which I did because it was misunderstood when I asked previously). As I've mentioned above, I was informed that updating to 5.0.1 would '''require''' me to '''delete''' the current version and then install the new one. And doing so will involve losing all my bookmarks. I guess I should have been more specific and detailed there. By losing, I didn't mean losing them forever. I'm aware that they're secured in some place and deleting and installing the software doesn't harm its existence. What I meant that if I install the new version, I'd have to delete the old one. And after installing the new version, I'd have to transfer them (bookmarks) back from wherever they are. Get it? When it updated from 3.6.9 to 3.6.13, and from 3.6.13 to 3.6.18, I didn't need to follow that process. They were already present on their own.
BTW, I'm having no problems with 3.6.18 but after learning about the existence of version 5.0.1, I'm a bit too eager to lay my hands over it.
Thanks for your help; hope this wasn't extremely long.