From MozillaZine Knowledge Base
This article deals with Firefox or Mozilla Suite/SeaMonkey not remembering user names and passwords you enter. If Thunderbird does not remember passwords, see Password not remembered - Thunderbird.
- If Firefox 3 does not automatically enter previously saved passwords when logging into websites and new passwords can't be saved, this can be caused by a bug in Firefox 3.0.2 that affects users with password data (login, password, or domain) containing non-ASCII characters.   If updating to Firefox 3.0.3 (or later) does not resolve the issue, see this forum topic for a solution.
- Firefox 3.5 and later won't offer to store a new password if you are in a Private Browsing mode session. One way to enter Private Browsing mode is if you select: Tools > Options > Privacy > History: Firefox will: "Never Remember History". You also see a, possibly grayed, "Tools > Stop Private Browsing" menu entry if you are currently in Private Browsing mode. To leave permanent Private Browsing mode go to "Tools > Options > Privacy" and choose the setting Firefox will: Use custom settings for history and uncheck "Automatically start Firefox in a private browsing session".
Password Manager settings
You must have the "Remember passwords" feature enabled for the browser to remember them. In Firefox 2.0 and above, make sure Tools -> Options -> Security / Passwords -> "Remember passwords for sites" is checked.
Additionally, you may have once told the browser to never save the passwords for the site. In Firefox 2.0 and above, check the list under Tools -> Options -> Security / Passwords -> Exceptions.
Passwords cleared with Clear Recent History
Firefox's Clear Recent History (previously named: Clear Private Data) feature may be set to clear your passwords when you close Firefox. Make sure that Tools -> Options -> Privacy -> "Clear history when Firefox closes" (previously named: "Always clear my private data when I close Firefox") is unchecked, or that under Settings, "Saved Passwords" is not checked.
Sites prohibit password saving
Some websites, for example Hotmail / MSN / live, GMail, Yahoo! Mail / flickr, and some banks, specify that your user name and password should not be saved. You can override this behavior.
Using a bookmarklet
- Bookmark the "remember passwords" link on this page.
- Go to the log in page of the site you want to save your details.
- Enter your login and password, but do not submit.
- Open the bookmark you just made. (Nothing will appear to happen).
On subsequent visits, your user name and password should be filled in. If that doesn't happen then see Password only filled after entering user name.
Firefox doesn't fill the user name and password if you have multiple names and passwords stored for a website (domain).
If the user name field is not empty then you may need to use the Delete key to clear the user name field and then the Down key should open the drop down list.
The "remember passwords" bookmarklet linked above may not work on all sites.
Here are some alternate versions that you can try.
Create a new bookmark and paste the code given below in the location field of that bookmark. 
- This is a variation of the original bookmarklet from www.squarefree.com with formatted output:
- This version works with frames, but doesn't give feedback, so you do not have to close the pop-up alert:
- This version gives feedback if you run it a second time:
Using an extension
Even after use of "remember_password" bookmarklet, and even deleting cookies for a site or other means above, you may still experience some difficulties with some sites.
- GMail/Google where after double-clicking on the userid and selecting your userid, it does not fill in the password. Choose to try another id, this time when you choose your id, it should fill in the password as well. You can only be logged into one Google account at a time. Google cookies are unique to each userid and to not signed in (no userid) within your profile. Cookies for GMail (mail.google.com), search on "@gmail".
- Yahoo remembers your password for two weeks so you think everything is fine then it fails. You may have to delete the cookies for Yahoo, use the remember bookmarklet and wait two weeks to see if you were really successful or not.
- Bank sites, financial sites, and other sites that have your social security number and financial transactions, and can transfer money do attempt to protect your password from being saved, you should not try to bypass such features that are designed to protect you and other customers. There are passwords you should just remember and not store them anywhere in, on or near your computer.
Likewise, the following software settings may impact your experience.
- Registry Mechanic (v. 10.0.0.143) might be configured to clear all browser data. To change preferences open RM, click Clean your Disks then click Browsers to access the settings for Firefox (and all browsers).