That is exactly the imac that I had except I had the 3.2 ghz i3. I even switched it to Windows 7. I lost it though when the graphics card went out. If graphics starts getting flaky be prepared to replace the graphics or lose the computer. I looked into a compatible graphics care and on ebay and they where $75-300 USED.
Not the end of the world if it happens. I got it for free as a hand-me-down and I don't use it much because I find Mac OS X relatively boring and I've got better options for running Windows.
It's the PowerPC macs and, if I can get some, the 680x0 macs that I care about... hopefully, by the time something goes bad with my Power Mac G4, I'll have had time to practice with my through-hole desoldering tool, save up for the hot tweezers on my wishlist (for desoldering two-lead surface-mount things like those capacitors Apple likes), and practice with those.
I'm still using 7 but I'm considering 10 as the primary OS on a second drive if I can get a copy. I've tried Linux and it just frustrates me. My latest attempt is with elemental OS and although I love the look it seems harder to install software than any other distribution I've tried. I can't get Firexfox esr installed. The instructions I've found fail because they don't allow using repositories. I don't think that I'll ever be able to depend upon a linux os.I don't know how it is now but I found mac more user friendly than any other Unix based OS that I've tried. I haven't used Mac though since High Sierra.
I've been daily driving Linux since 2002, which says a lot of what I was willing to tolerate back then, and how used to it I am now.
The only distros I've used are MandrakeLinux 10.x, Gentoo, and various versions of Kubuntu, Lubuntu, Debian, and Slax, so I can't speak to your distro specifically, but feel free to ask me if you're having trouble. I'll probably generally know.
In the case of Firefox ESR, which installation method were you trying? You mention repositories, so I suspect it might have been a set of Ubuntu-specific instructions.
Normally, if I'm not just installing Firefox through Flatpak, I'll just use Mozilla's download which just unpacks into a folder and runs, like a zipped/non-installer EXE on Windows would.
Given how reskinnable the desktops are, and how most distros offer most desktop environments as secondary options, I generally pick a distro based on software support and then adapt the UI, rather than the other way around. After all, the package repository is the core of what a distro
is.
Mac OS X and beyond are definitely more geared toward less experienced users, but it's also much more rigid, which means you'd better already like the choices Apple made for you.