Note that you can hover a tab with the mouse to see the title in a tooltip. You can set this tabMinWidth pref on the about:config page to a higher value.

• browser.tabs.tabMinWidth = 250

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Here's something else you could consider. Firefox displays an icon at the end of the tab bar when the tabs are "overflowing," meaning that not all the tabs fit on the bar and scroll buttons have appeared at the left and right of the tabs. This list is very handy for seeing long tab titles and switching tabs. It looks like:

That's not the full title in every case, but it usually is enough to distinguish between pages.

There is a setting to enable that button to appear even before the bar overflow. Here's how:

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

(2) In the search box in the page, type or paste browser.tabs.tabmanager.enabled and pause while the list is filtered

(3) Double-click the preference to switch the value from false to true

This might not change windows that are already open, but should change all new windows (or all windows the next time you exit/restart Firefox).