The browser.history_expire_days and browser.history_expire_days_min prefs have been removed in Firefox 4+ versions and replaced with a single places.history.enabled pref to remember history or disable the history.
You can reset no longer used browser.history_expire_days* prefs used by older Firefox versions on the about:config page and they will be removed on the next start of Firefox.

Current Firefox versions do not have a time limit for the history.
Firefox determines automatically how many pages can be kept without affecting the performance.
You can see the current value via the read-only pref places.history.expiration.transient_current_max_pages on the about:config page.

See Marco Bonardo's blog about this:

• http://blog.bonardo.net/2010/01/20/places-got-async-expiration
• https://developer.mozilla.org/en/Places_Expiration#Preferences

If history gets removed automatically then you may either reach this maximum page limit or you have cleanup software that remove it.

I don't want to remove any of my history, i want to keep all of it. Why does firefox prevent me from doing that now? And why would history effect performance when all its doing is keeping tack of what i've visited in the past?

If you visit a website then Firefox needs to check the history to see if you've visited the website before and adjust the visit count and this is also needed for things like suggestions in the location bar drop down list.
Having a large history would take time to process the history to do a specific query.

That seems unnecessary, why check if you've been too it instead of just adding +1 to the visit count.

Seems like a very inefficient way of programming.