Was this the built-in "form history" feature where a drop-down appears with previously filled entries saved by Firefox? Or was it something more sophisticated that could have been provided by an add-on?

To make sure Firefox's form history feature is turned on, you can check the Options page, Privacy panel. Either:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences

In the left column, click Privacy. Then on the right side, change the selector to "Firefox will: Use custom settings for history" so you can see the detailed settings.

Is the box for "Remember search and form history" checked?

This was not a add on. I had already tried was you suggested and that didn't work. Thanks for trying

Could you check whether you still have form history entries or whether they might have been erased? This add-on provides a way to view and edit them:

https://addons.mozilla.org/firefox/addon/form-history-control/

Note: In order to use that, you might need to disable Firefox's "multiprocess" feature.

Recent versions of Firefox are migrating more users to a "multiprocess" architecture where the user interface and the web content are in separate processes to improve stability. You can check whether your Firefox is using that feature as follows:

Are you using Multiprocess (e10s)?

Multiprocess creates a second firefox.exe or plugin-container.exe process to isolate the web content from the browser UI. You can check whether you have this feature turned on as follows. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s.

If you are using e10s:

To help evaluate whether that feature is causing problems, you could turn it off as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. That provides backwards compatibility with older extensions.

You can also try a Form Filling add-on.

Go to the Mozilla Add-ons Web Page https://addons.mozilla.org/en-US/firefox/

(There’s a lot of good stuff here) and search for what you want.