Well, thanks in part to the suggestions made by the commenters to my earlier post, I’ve managed to get Firefox set up to work almost like Opera, and in some ways even better. I’ve only had to make a few minor adjustments to my habits, like using Ctrl-click to open a new tab rather than Shift-click; and there are a few things I have yet to get used to, like the tabs being on top instead of on bottom, or the Find in Page box being in the lower left and opened by Ctrl-F. One drawback I’ve just discovered, though, is that I can’t seem to reduce the tabs in size within the window; if I want to have two half-size pages side-by-side, I need to open them in separate windows. But that’s a minor adjustment and might actually be slightly easier.
As for the Thunderbird mail client, it seems to work after all. I’ve realized that the main reason it didn’t seem to be getting new messages consistently is that I often mark them as read or delete them on my smartphone before Thunderbird gets around to checking for them. I didn’t realize until yesterday that when I do that on the phone, it changes the messages’ status on the mail server itself. I’m used to my old Eudora client program that used POP (locally downloading and working with mail) rather than IMAP (interfacing directly with the server). But Thunderbird’s already notified me of two incoming e-mails this morning, so I know it works. I’ve now got it set up to interact with both my mail accounts, and I discovered that I could use it as an RSS feed reader as well, which lets it take over the one last Opera 12 function that Firefox didn’t seem equipped for. (Yes, FF has Live Bookmarks, which does something similar, but Thunderbird’s format is closer to what I’m used to from Opera.) I preferred Opera’s arrangement of three parallel vertical columns, since Thunderbird puts the pages in a fairly short window underneath the list of entries; but it’s easy enough to click on the link and read the message on its original page in my browser. So it’s a minor adjustment.
So I’m finally back to a place where I only need to have one browser open as a matter of course rather than two — and just a few days ago, I was afraid I’d have to get used to switching among three browsers to do different things. So I’m definitely glad I managed to sort this out. And thanks to the commenters for the helpful suggestions.